Personally I started to delete some of my comments with the newest Sandbox because of this specific issue and my personal habit of always leaving lots of very long comments. But I don't think it's necessary to do anything special.
If you want you can delete your comments to make the Sandbox a bit easier to use for others. And everyone else can remove theirs, too. But requiring everyone to remove their comments is a bit much. And writing comments to others that they should please remove their comments is... weird. We also quite often have problems with people abandoning their drafts and keeping an eye on which drafts are still worked on and which are abandoned. Keeping track of every single comment by personally contacting the user doesn't seem useful...
Flagging for moderator attention would be possible, but I think it's already a bit much that moderators are so often tasked with removing abandoned drafts. Obviously only a mod can tell you whether it's a lot of work for them or whether they can do it together with removing the draft for example.
All in all I'd say this is not really a problem. If you want to be considerate you can occasionally delete your comments under drafts from others when they have served their purpose and you can delete your own comments when posting on main, but shortening long drafts and deleting them is far more important in my opinion. The red background is enough to easily see that a draft doesn't need help anymore and that's the main reason we established the "Keeping the Sandbox clean" rules.
And if the Sandbox gets too full we can simply create a new one. The "worst" thing about the last Sandbox was that it was a bit slow to load. I for example looked through the Sandbox a bit less, but it's not like the Sandbox needs to be checked every five minutes by everyone. Having a slightly slower Sandbox is a nuisance but not something that requires immediate attention and not something that needs massive amounts of additional work.
Again, the main reason for "Keeping the Sandbox clean" is to make it easier to keep track of which drafts still need help and which don't.
Just as a general thing: if you are thinking about flagging a lot of comments you should use a custom mod flag and simply state why and which comments you think need to be deleted or moved. You don't need half a dozen flags or more that all say "No longer needed" when you can simply say "The comments are obsolete now that I have posted on Main. Can you please remove the comments?" or for example on Main "The comment discussion is drifting far away from improving the question and it's mainly two/three people arguing. Can you please move the discussion to chat?" - again, mods probably know far better what they would like, but in general one clear message is easier to act on than lots of basically unconnected flags.