[The on-topic bit is at the bottom...]
I ended up using Canon "accidentally", around 20 years ago, when most of my Pentax kit was stolen, including the digital body.
The insurance replacement was a 30D, and I think it was probably the nicest to use of the three Canon bodies I've owned, because it was straigthforward. But the sensor became very noisy, and I longed for a full-frame body, too.
I replaced it with a 6D, eventually running Magic Lantern (open-source 3rd party firmware). It was OK to use and I got some nice shots with it.
I finally upgraded in 2024 to a mirrorless R6 Mk2. I have to use all my old EF lenses with an adaptor, but they still work well. It is, however, really complex, and I frequently find that key functions are buried down in the menu structure, unhelpfully. There are also too many controls for my right hand, especially ones that should need a thumb to operate them. My thumb cannot do this properly, and many shots have been missed, simply because of fumbling about. Compared to the original 30D, the camera also eats batteries, adding to the weight of kit I need to lug about.
So, to the important choice: lenses. I only have three in regular use. The zoom is a 'professional' 24-105 (RF f/4 IS), and my wide is a fully manual Laowa 15mm f/2 with an RF mount. I've kept my EF Canon 300 f/4, with a 2x converter and an EF-to-RF adaptor. The 300 is great, and even adequate for a lot of candid stuff, and wildlife point-and-shoot applications. It cost thousands less than the f/2.8 version, but the image quality is excellent, and it's a delight to use - one that Canon got right. It also holds its value on the secondhand market (not that I'll ever sell it!).
Based on only the 24-105, I have to say though that I don't like the RF lenses: cheapness shines out of it, and optically it's not all that good for quite a lot of extra money over the 'consumer' lens. It's basically the EF version reworked into an unnecessarily big body (the RF format should allow lenses to be physically smaller..
It also has an infuriating third ring, that can be defined in firmware. Initially, I thought, "Hooray! I can have an aperture ring back!" Yes, it will do this, or several other functions, but it's in the wrong place on the lens, it cannot be easily felt with fingertips (there are no click stops either), and the lens hood (when stored backwards) actually covers it up. So it's a gimmick.
In contrast, the Laowa 15mm is an absolute delight, and complements the mirrorless camera body really well. The camera will do 'focus peaking', which was a feature found originally only on broadcast TV cameras, and is a focusing aid in the viewfinder (DSLRs cannot do this in stills mode, but some can in video). When the lens is fitted, the camera defaults to peaking (it thinks there's no lens fitted at all!), which makes focusing very easy indeed. ObviousIy I love using it.
I was nervous about mirrorless. On balance, I like it, now I've got used to it. Focus peaking especially is very useful, so is the ability to see the 'taking aperture' in the viewfinder, and other handy stuff like a built in 'spirit level'. Also helpful is the 2 second delay timer - push the shutter first, then concentrate on holding the camera still! Lots of landscape photography needs long exposures, and sometimes I can't use a tripod.
But as a general rule though, the viewfinder tends to be a bit cluttered, with many silly icons I can't turn off (some, OK, but not all of them). 90% of my photography is in aperture-priority or manual modes, so there's a limit to what I need to see on-screen.
I haven't played much with the video side, but I have had good results with it. The body is in the semi-pro category, and has a bunch of features which actually do add value in this area, including a clever (undocumented!) ability to jam-sync timecode (invaluable for multi-camera use).
Given my age and funds, the R6 Mk2 is likely a forever purchase. I can live with it, but if someone let me go back in time to my Pentax LX and bulk-loaded film etc. I'd probably enjoy my photography more! It's telling that neither of my favourite lenses are current Canon RF-series ones.
Replacement choices for the 400D
I agree with others: the 7D was an excellent model. It would fit the bill. It's classed as semi-professional and was very popular for video use (amongst Canon users). If you want to go full-frame, the 6D is nice and pretty straightforward, and inexpensive secondhand. Anything in the 5D series is also very good, but heavier and bulkier (that puts me off - one reason for my choice of the R6 II was its 'compact' size).
The other thing is the range of lenses available: Canon has tried to lock down the recent RF mount system as much as possible. I didn't know at the time, but it seems that my Laowa 15mm is one of the few 3rd party lenses available in the RF mount. That's probably because it has zero electronics within! In contrast there's plenty of choice in the older EF mount, you can use them on an R-series body (with an adaptor) too.
Buying digital secondhand isn't the nightmare it was. Yes there's eBay, but there are also other decent camera shops like MPB (online) and Wex (my camera came from them), both of whom will warrant the secondhand stuff they sell. Browsing MPB, in particular, will give you a good idea of prices.
And keep your lenses! Two reasons: you're used to them, and if, like me, you're fussy abut the glass, they're a known quantity. That said, in EF there are some excellent Sigma and Tamron lenses (my previous ultra-wide was a Sigma 12-24 EF zoom), and they're available secondhand at quite modest cost now. If you stick with EF, there's a lot of prime lenses out there with excellent performance (even some of Canon's range!!), at modest cost. I even have "naughty" adaptors to let me use my old Pentax K-series lenses on Canon EF bodies (you can do this with the EF mount but mounts from other camera manufacturers are smaller diameter, so an adaptor cannot be fitted).
One word of caution regarding zooms: even though manufacturers can now grind aspherical surfaces fairly easily, zooms remain hard to make, especially in shorter focal lengths. That's if you want optical quality.
There's two trade-offs: Firstly, aperture - zooms are usually slow lenses - they have to be (it's almost a law!). A 'fast' zoom (heading towards f/2.8 or faster) WILL be optically poor when wide open, and have 'quirks' over its zoom range. Secondly, whilst it's possible to have zooms with a huge range of focal length, they're usually both optically pretty poor and need a lot of moving elements within them (thus tend to be big and heavy). There are many cheaper and lighter weight forms of male jewellery available, such as gold bullion.
I note in passing that a sports 20:1 (or even 100:1) zooom lens for TV can cost $100k. Most TV companies rent. The reason is the low production volumes and the sheer difficulty of making the optics work*.
I hope the above is helpful. Wander around the MPB site and see what you can find...
E.
*Zoom tracking, meaning the lens staying in sharp focus whilst being zoomed, is essential for TV lenses. It's extremely hard to do with extreme-range zooms, which means a lot of fast glass, a physically huge lens package, and a mortgage. Stills cameras 'cheat', by grabbing autofocus just before firing the shutter, so stills zooms don't have to track in the same way. It makes them loads cheaper.
) but I ran this through Google from doing the sensor clean "in camera" and also Google mentioned a "wet clean" using swabs. I'm guessing you haven't tried a wet clean?
. That line hadn't stood out to me... Maybe I should edit my post - and delete all those photos as they're not of any use... Sorry that I'd not seen the thin black line and it was the other bits that hot my attention. Obviously now I realise it's the glass... 
Buried settings would annoy me. It's one of the reasons I haven't swapped my car as the trend towards scrolling through menues instead of buttons for the simplest of tasks is irritating and dangerous.