The A7 III is the more complete mirrorless camera package, with the latest Sony AF system in tow. The A7R is better suited for highly detailed work, can take more pixel-rich photos, and can better assist you due to the better EVF and resolution.
If you’re a landscape, fine art, or still life photographer or you’re in the realm where pixel quality and image resolution is a major key, the A7R is the more appropriate camera. Things like 4K recording and the Pixel Shift feature would just be welcomed additions to your tripod-assisted, landscape shooting.
However, if this is your first mirrorless, or you need to upgrade from prior Alpha cameras, buy the A7 III. Especially if you shoot a bit of everything and everywhere (sports, subjects, landscapes, urban). The larger image buffer gives you more flexibility in the moment, with as many AF points as the sports-oriented A9. It’s also slightly lighter and of course $1,200 less expensive.
Ultimately, you can have fantastic all-around performance with the A7 III or megapixel overkill with the A7R III. Both are great cameras, but one is far more suitable for the niche of professionals who specialize in detail-oriented photography, while the other is going to be on many new photographer’s wishlists.