*G is very useful for someone who would use LHR frequently.

The problem is from EDI you have no good way to get to LHR, with no domestic *A flights you'll have to allow 7-8 hours for train or 4-5 hours for a BA flight.

As you say, to stick with *A you're likely to be seeing FRA a lot and I hate FRA.

Regarding priority security, while I've only used EDI once since before covid, the priority queue seemed to be the same as the family queue. Anyway there's no issue if both parents will be *G.

I think you will have to go with easyJet etc if there is a direct option to where you want to go. Pay for the extras if necessary. Use *A when there is a sensible schedule. Otherwise you will end up letting the airline determine your destination - which is fine if you are aware that is what you're doing.