Adding my two cents here... you really need to be careful with diffraction on that 61MP sensor. I would suggest looking at technical MTF charts on sites like LensRentals or the reviews from Phillip Reeve to see how glass actually behaves at high spatial frequencies. Basically, if you stop down past f/7.1, you are gonna lose that biting crispness.

• Make sure to test your copies for decentering. That sensor shows even the tiniest lens element misalignment.
• You might want to consider the Sony FE 20mm f/1.8 G. It is technically one of the sharpest wide angles for that sensor, often beating G Masters in corner resolution.
• I would suggest the Sony FE 50mm f/1.2 GM if you want the absolute highest resolving power Sony offers, though it might be tight for landscapes.
• Look into the Sigma 35mm f/1.4 DG DN Art too. The optical design is very clinical and might solve your edge issues.

Man, I feel your pain. I took my a7R V to the Dolomites last year thinking my old setup would hold up, but that 61MP sensor is brutal. It shows every single flaw. I actually had issues with two copies of that 35mm GM before I gave up because the corner smearing was just unacceptable. Its frustrating when you drop 4k on a body and the glass cant keep up. For Iceland, you need stuff thats tack sharp edge-to-edge. Here is what actually worked for me after a lot of trial and error:

• Sony FE 50mm f/1.2 GM
• This is probably the sharpest lens I have ever owned. It handles the high res sensor easily.
• Sony FE 24mm f/1.4 GM
• Much better corners for landscape than the 35mm in my experience.
• Sony FE 135mm f/1.8 GM
• If you want scary sharp results, this is the gold standard. Tbh, if your 35mm is soft, you might just have a dud copy...