I think your situation is a classic conundrum as it will lead to any number of unexpected twists. I find these fun be nerve wracking at times.

First, I agree that for your stated preference of landscapes you're best off with a full frame system. You have a great list of lenses but I must admit that I'm not sure how many are full frame and how many crop frame. That could help your decision because switching to mirrorless is generally best getting lenses made for the new body. The D810 by all reports is an excellent camera and will probably meet your needs if you choose to stay with that. (The D7100 is a decent backup if you want one.)

On the other hand some of us like to push on to new things that take us a bit out of our comfort zone but may lead to improved pictures. I recently switched from my Nikon D7100 and crop lenses to a Sony a7iii with a Sony 24-105 full frame lens. The Sony menu still drives me crazy but I am slowly mastering it--at least for my purposes. The IQ and low light capabilities of the Sony are really good (although I'm not sure how that would compare to your D810). In general I'm very happy with switching and am enjoying learning a new system. If you like challenges and want to be on the cutting edge moving to mirrorless full frame is probably a viable idea. Good luck and enjoy.

Then stick to a full frame camera. There is nothing wrong with a Nikon 810, it's a great camera.

My personal preference is to stick with DSLR, mostly due to the optical viewfinder. If the electronic viewfinder doesn't bother you then your decision becomes a lot harder.

The weight difference is minimal but might be noticeable using smaller lenses. Carrying extra batteries might also mitigate that weight difference. Having to also add a converter to adapt the lenses would also add weight.

If it was me, I would keep the camera and invest in a new lens or two and maybe a camera bag or tripod. A new camera won't take better photos, but a new lens can.

Good luck.