If they like dressing up, then I would definitely get them more than just shirts. For myself, it wouldn't be any fun to do something for Halloween and not wear a costume. But I'm much older than your boys, and also I am a girl.
If they want to dress up, then the Target idea is good. But since they are boys and not princesses, I'm assuming a lot of things they would want to wear is probably small enough to be easily packed anyway. Obviously a Mike Wazowski costume or something would be hard to pack. But anything that's just clothes and no mask (pirate, cowboy, sports star, etc.) wouldn't have to be heavy or bulky to pack (or to wear), IMO. It's just like packing an extra kids' outfit, which could fit into the top of your suitcase in a pinch.
Comfortable shoes are probably the biggest key, but most kids wear their regular shoes with their costume so I don't think it's a problem.
Some hotels accept deliveries on your behalf in advance, so you could also mail the costumes to yourself if need be. That's what I would do if I were flying and my costume wouldn't fit in my suitcase (but I have some biiiig costumes).
If they don't like dressing up, then the shirts are fine. It really doesn't matter whether other Disney guests feel the costumes are "low effort" or not. It only matters that you and your kids are comfortable* and happy.
(*I don't really care if my own costumes are comfortable or not, but again, I'm much older than your boys, and a girl.

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