I may be cynical, but I suspect the main reason each brand has different mounts is to protect their profits. Even so, I'm not sure how much I would trust an adapter

"Switch to full frame" -- Unlike Nikon, where crop-frame lenses can sorta be used on a full frame camera, Canon does not (as far as I know) allow EF-S lenses to be used on a full-frame camera. You would have to get new lenses and then all bets are off.

I have a humble Canon Rebel T3 and I can confirm it does tell you how many images it can store with the current image settings.

I would suggest shooting both raw and JPG (assuming you think the card(s) will be able to hold enough images). The reason is that the JPG image will be relatively small, and even though RAW gives better quality and versatility, JPG gives a picture you can use immediately.

If you find you never want to share a picture quickly and easily, cut out the JPG.