Screen adhesive re-use

I'd say it depends on how well the glass comes off. I had an iPhone X that I had just finished working on but banged my head on the table when I realized I'd forgotten to reprogram the True Tone data. So I opened it back up and found that the precut adhesive had stuck on the phone so it was still completely intact and undamaged. That being the case, I decided to reuse it and haven't seen any problem as a result.

So my take on this as it applies to you is, if the display comes off cleanly and doesn't tear or stretch the precut adhesive, you can probably get away with reusing it. If it sticks partly to the screen or tears / stretches when you get the screen off, then it's time to replace it.

One thing you might do when you put the phone back together is to reheat the adhesive once the screen is on, then place it face down with some weight on it. That'll soften the adhesive and the weight will help seal it. Hopefully that'll help compensate for not using a new sheet of adhesive. I usually go over the phone for about 3 minutes with a hair dryer, then I have a motorcycle battery I set on it for a half hour or so.