If you choose to go Sony APS-C mirrorless, you can of course continue to use your same lenses if you go full frame later (albeit obviously with a changed effective focal length). We have both in our household, and merrily exchange lenses and accessories between the two of us. You also have to option to use lenses form almost any other manufacturer via an adaptor, in many cases retaining full autofocus. That's a plus point of mirrorless, the fact that you have the available length that the mirror box used to take up, for lens adaptors. Obviously for a lens to be usable on both formats, it needs to be a full frame lens!
On the subject of optical viewfinders vs. electronic ones as per mirrorless cameras, I recently used someone else's APS-C Canon SLR, and thought the viewfinder was horrible, with uneven illumination, and it looked sort of artificial. I've got used to the brilliant viewfinder of my A7Rii (which is admittedly well out of the price range wanted here) but others must be similar in quality.
Its worth noting that for any given format, mirrorless lenses are not magically smaller than DSLR lenses, the size reduction is only in the camera body!