Hi Ray, taking more time to think this one through. In my case the SS plate was visible in the pool as I used the vinyl liner as a gasket. That meant I went for a thicker plate so I could get a good smooth radiused edge with no danger of anyone cutting themselves.

I maybe wrong but it sounds like you are going to repair from behind the liner therefore not on show so thinner would be more desirable?

I did one wooden structured pool from behind the liner and I used 1mm on that which still showed a slight bump but the owner was happy so if that is your plan then yes 26 gauge SS sounds fine.

Sorry for adding any confusion, maybe a picture when you have finished as it's a common issue affecting owners. If there is a shop near you that can laser cut and make the holes that would be worth doing as the thinner SS plate maybe even harder/dangerous to work with.