As to resale value, keep in mind that camera bodies are primarily computers, and since technology advances quickly, used computers lose their value quickly. With first-gen bodies it happens even faster. (And the R seems more beta than first gen so add that into the depreciation mix too.)
I think the R5 will hold its value better than the R. But once the R5 mark II comes out, the R5 classic will lose about 40% of its original price. And when the MkIII gets released, they'll be giving away the first-gen versions. Look at used prices for first- and second-gen 5D bodies to see what I'm talking about.
Glass, on the other hand, holds its value very well although with lenses getting more computerized that might change too.
For me, photography is just a hobby and I generally outfit my hobbies with used gear. I probably won't go mirrorless until I start seeing Mark II versions hit the used markets. I'm just now looking for a used 5D MkIV. Very reasonably priced these days.
It is but it took 4 plus years. The mirrorless market also did a number on its value. I sold mine two weeks ago for a lot less than I wanted but had to in order to offset the cost of the R6.

