I agree with the premise serve +1 and return +1 are the most important shots.
However, serving a bucket followed by a hand feed doesn't mimmck this that much
Firstly, new fast balls are used in matches
2ndly, you don't get a hand feed in a match
3rdly, where's the return being practiced here?
1) Doesn't really matter.
2) Again, doesn't matter. Just need to train the ability to recover your balance quickly and redirect shots to the open court. You can do a LOT with hand feeds. You can also have someone at the opposing baseline drop feed balls to you.
3) You have a server stand on the service line with 2 balls, serve the first and drop feed the second. Different drill.
Doing it as serve and returns being played allows you to train your ability to read the return, but hinges on both of you consistently getting serves and returns in (which is unrealistic - 70% first serve success rate and 70% return success rate means you see a +1 less than 50% of the time). Yeah, you can change it to second serves, but then it becomes much more return favored unless you both have very good second serves. Yes. the drill can be done with average second serves, but realistically the result should be a neutral rally more often than not.
I agree with you that drilling crosscourt has limited utility. But I’m not quite sure why playing sets doesn’t make you… better at winning sets? Aren’t you drilling your serve plus one and return plus one then?
Playing sets makes you better at handling what your opponent gives you with the toolset you have on the day. It's live problem solving and adaptation. And again, it's an issue of time efficiency. You can practice with someone trying to hit a winner on every shot and pick up balls every 10-15 seconds, or you can rally cooperatively so you guys can 30-40 balls a minute for half an hour and get a good workout. You can learn to hit overheads exclusively from matches, but if you do cooperative overhead/lob rallies with a partner, then you can hit more overheads in 5-10 minutes as you would've in the past year or more of tennis matches, and you can quickly go from being bad at overheads to making your opponents afraid of lobbing you.
I've seen players improve faster and more drastically with cooperative hitting than by doing the "same old same old" hitting and playing some sets. Match play matters, but so does goal-oriented practice.