I tried to think of how I'd look at our priorities in the summer, but I'm struggling because so much of it depends on if/how we decide we want to play.
We talk a lot on here about whether player X, Y or Z is good enough, but I think a bigger problem we have is that we have a squad that clearly haven't been assembled to fit a certain way of playing. Letting Travis go on loan was a shocking decision, but then there's no point keeping him if we want our midfielders to play like Wharton. Similarly, there's no point in keeping Gallagher if we're not going to play to his strengths (which then wouldn't really suit Dolan and Markanday, etc.). I don't especially care how we play, just so long as we start recruiting in a way which is a bit less scattergun and more strategic.
Having said all that, I think you're completely right about needing a new keeper, and we're at least one full back short. I don't hate our midfield options if we're going to be playing a more Travis-friendly game. We need another centre back if we're playing 3 there, but Hyam Wharton and Carter are all decent options in a standard set-up and I wouldn't be spending our limited resources on an upgrade. The big question marks are in attack. We definitely need more quality there to either help or replace Szmodics, but who we go for (and who we let go) for me is a function of how we want to play.
Must say that you rate Siggy more highly than I do. He has ability, but he lets far too many games pass him by. Beyond his fairly average goal return, I don't think he's contributed anything really.