When I travel I take my APS-C camera - a Canon SL ... (
A previous response is correct.... The Canon SL2 (also called 200D) uses a Digic 7 processor.
You don't say what you hope to gain with a camera "upgrade". And you don't specify what Canon models you've looked at.
Just for example, the Canon SL3 (250D) is one of the models that uses the Digic 8, one generation newer than your camera's processor. Some of the more recent Canon cameras have had Digic 9 and even Digic X (10) in a few of the latest releases (EOS 1DX Mark III, EOS R6, EOS R5).
The primary difference between those two models, between Digic 7 and Digic 8, is video related. The SL3 and Digic 8 can shoot 4K video, while the SL2 and Digic 7 at "limited" to HD video. There is little other difference between the cameras (both of them are significantly more full-featured than the original SL1/100D). Both cameras shoot 24MP still photos. Both have rather simplistic 9-point AF systems. Both have the same maxiumum 5 frames per second shooting rate. Both use the same SD memory card and both use the same LP-E17 battery, although the SL3 is supposedly more power efficient and will get more shots per charge (over 1000 vs about 670, using CIPA rating method).
Another difference is the Digic 8 and later processors now produce a "CR3" RAW file, instead of the "CR2" of the earlier models. This can require a software update, though you would probably not need it since your Adobe LR and PS subscription keeps you up to date.
Digic 8 claims some processing power increase, which might translate into slightly faster AF. However, both the SL2 and SL3 use a fairly old design, 9-point AF system with only a single dual axis/cross type AF point at the center. You would be more likely to see AF improvements comparing the cameras in Live View or video mode, where the dual pixel CMOS embedded in the image sensor is doing the work in both cameras.
Compare Canon SL3 (250D) to SL2 (200D): https://cameradecision.com/compare/Canon-EOS-Rebel-SL3-vs-Canon-EOS-Rebel-SL2
But the lens is another potential limitation of AF performance. For example, you would probably see more responsive AF from Canon's latest EF-S 18-135mm IS "USM" and EF 70-300mm IS USM "II" lenses, both of which boast the "Nano USM" focus drive system. They claim Nano USM gives you best of both worlds... the smooth, silent focus of STM as well as the speed and tracking ability of USM. I have no hands-on experience with it, but would wager the Tamron 18-400mm's "HLD" focus drive is nowhere near as quick as Canon's Nano USM. HLD sounds to be an optimized form of "micro motor" drive, which is the slowest type. (Canon uses micro motor too, in their most budget-oriented lenses... the ones not marked "STM" or "USM".)
What are you trying to achieve? What do you think the new camera should do for you, that your current camera doesn't do?
The Canon SL2 is one of the smallest and lightest DSLRs anyone has ever made... great for travel and not a lot larger or heavier than most mirrorless cameras. If size and weigh were your concern, you'd have to look to mirrorless to do any better... and if you want a viewfinder on the mirrorless the size/weight savings will be minimal.
Compare M50 Mark II with SL2: https://cameradecision.com/compare/Canon-EOS-M50-Mark-II-vs-Canon-EOS-Rebel-SL2
Even less difference in size and weight with the M5 that I use.
The electronic viewfinders (EVF) of these mirrorless cameras have some useful features that aren't available in optical viewfinders (OVF) like your SL2 and other DSLRs use. EVFs can give you exposure preview, can be very helpful shooting in low light conditions and can have manual focus assist features. However, they also drain the camera's batter much faster. Typically these mirrorless are rated to get about half as many shots per charge as your camera, even though they may use the same battery.
To get higher resolution you'd need to look at either 90D (DSLR) or M6 Mark II, both of which offer 32.5MP. That's higher resolution than any APS-C camera offered by anyone. The next closest is just 26MP and even other Canon APS-C are 24MP (like your camera). Heck, 32.5MP is even higher resolution than most of Canon's own full frame
The problem is the 90D is significantly bigger and heavier than your SL2: https://cameradecision.com/compare/Canon-EOS-90D-vs-Canon-EOS-Rebel-SL2
So, what about the M6 Mark II? Well, it does save a little weight and is a bit more compact... but it lacks a viewfinder. Everything is done using the LCD screen on the rear. There is an accessory EVF available for it (fits into the flash hot shoe). But then you're back to square one with size and weight.
Another problem with all the M-series cameras is that they use the EF-M mount lenses and there is quite limited selection of those. Though some are quite good, most are zooms and all use quite/smooth but less fast "STM" focus drive. (I use a small kit of four prime lenses with my M5: 12mm, 22mm, 56mm and 90mm.) The longest telephoto offered is Canon EF-M 55-200mm. Yes, you can fit your 18-400mm with EF mount to the M-series via an adapter. But, once again, that largely negates any savings of size and weight the camera itself offered.
The very high resolution 32.5MP cameras also are very demanding of top quality lenses. Any lens shortcomings will be very obvious. You might be quite happy with your 18-400mm on a 24MP camera, but not very pleased with it on 32.5MP (this resolution on APS-C is higher than any camera on the market offers. The same pixel density on a full frame camera would make it 83MP. Or a typical medium format camera with this density would be in the 140-160MP range. No full frame or medium format digital cameras offer these levels of resolution yet. Maybe someday they will. When they do, they'll also be very unforgiving of the lenses used upon them!
In the end, you might be best just keeping the camera you've got... Maybe just try some different lenses on it? If it were me, I'd want a wide angle. The Canon EF-S 10-18mm IS STM is compact, light and very affordable. If you don't use the longer focal lengths much, you might consider the EF-S 18-135mm IS USM as a general purpose, walk-around lens. If you need telephoto, the EF 70-300mm IS USM II is hard to beat for a reasonably compact, lightweight choice. If you like to do macro shots, the Canon EF-S 60mm f/2.8 USM is one of the more compact options. Of course, all these won't be as convenient as the single lens you carry now. They'll give you better image quality, though... plus, potentially, faster focus and better tracking of moving subjects.
Compare image quality Tamron 18-400mm vs Canon EF-S 18-135mm IS USM: https://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=1145&Camera=963&Sample=0&FLI=0&API=0&LensComp=1045&CameraComp=0&FLIComp=0&APIComp=0
Compare image quality Tamron 18-400mm vs Canon EF 70-300mm IS USM II: https://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=1145&Camera=963&Sample=0&FLI=2&API=1&LensComp=1077&CameraComp=963&SampleComp=0&FLIComp=1&APIComp=0