I wholly agree with what Raymond says, definitely go mirrorless, they are better in every way than DSLR's now and more compact, even the top of the range Sony A1 / A9iii are half the size of your 1d3!
I'm not completely up on Canon's range these days as I made the switch to Sony after the Canon 70D and a few other brands here and there. I currently use the a7iv, which would be the oldest model I would consider, if you can afford to, aim for the a7v, or even the a7cii, as these have AI autofocus modes now which the a7iv doesn't (though it's stiklll no slouch when it comes to AF, it's still better than most), (I'm not sure whether the a7r1v has the AI features either). Also, one thing to consider with Sony's line up is the model numbers, for example, the a7ii /a7iii / a7iv are all hybrid models, they have been designed with both photography and video in mind, whereas the models with an R in them (a7Riii / A7Riv) are designed with photography in mind so they have bigger sensors, faster shutter speeds and tools design to improve their photography abilities, whereas the S models (a7Siii / A7Siv) are desgined with video in mind, so they have better video recording capabilities (such as 4k120 less rolling shutter etc).
I settled on the a7iv as its the best I could afford at the time, (I got it for a real bargain price brand new), and video is something I wanted to get into as well as photography, but I still haven't gotten into the video side of things yet lol, I am completely satisfied with the camera though for photography though so I don't feel like i'm missing out.
Lenses...
Iff you're quite invested in Canon glass, just get the EF-FE adapter for now, that'll do you to start with, but Sony really do make some good glass. Again, what Raymond said is true about Canon, their RF glass is so much more expensive than their EF glass was. Sony is probably on a par, or just a bit more expensive than the EF glass was. I'm currently using 3 lenses (about to buy 2 more non Sony lenses too). I have the Zeiss 16-35 F4, Sony 24-105 F4 G and the Sony 100-400 F/4-5.6 GM. With Sony, G is their mid range glass and GM is their top of the range, howeverm the 100-400, for GM glass is unbeatable value compared to most of their GM glass.
Another thing to consider too, Canon, at this moment anyway, don't support 3rd party glass, so you can't use Sigma / Tamron etc glass with them, I think there are plans in future to allow them though, however, Sony have MASSIVE support from 3rd party lenses, Sigma / Tamron / Viltrox/ Laowa / TTArtisans, etc....etc, and there are some wierd and wonderful lenses you can get from these 3rd parties. Laowa for example, make some crazy lenses, there's the probe lens they do, they have some of the best macro lenses you can buy, (90mm 2x macro, 25mm 2.5-5x macro, and even the kit that gives you between 10 and 50x magnification, (Laowa Auragon set). There are tilt shift lenses, fisheye, so many, and they're so much value for money too. One thing of note though, With Sony 1.4 and 2x teleconvertors, you can only you Sony lenses, and then only certain ones at that too, but that's only something to consider later on I guess.
Anyway, that'll do for now. Hope what I've said is helpful and good luck choosing and enjoy shopping!
