Pologne, Benelux, Italian fall races or even Canadian - just to get the zip back in the legs and racing miles with low pressure. Hopefully helps build hunger for next season
I think the biggest problem is that for the moment he's stuck in some sort of limbo. Since joining TJV he has never really achieved much other than his Olympic medal. During the 2020 TDF we knew we couldn't expect the best Dumoulin, and while he was far from his best form, he wasn't bad either. Then he took some time off and came back with more hunger. But so far we haven't seen a glimpse of the GC rider he once was.
Is he lacking race days? Could he still be a viable GC option? Then he should get back in the saddle as fast as possible imho and try to get some races in those legs, and work towards the Vuelta. That way he could see if there enough improvement to be made by august/september. If he doesn't get those racedays now, then i fear he won't be ready for the Vuelta, and if he tries again next year, he will have lost an other year, with no idea whether or not he still has it in him. Or perhaps facing the same issue once again.
Does he even want to anymore, last year all signs pointed to 'no' but this year he seemed to want it again. But if they really don't know what caused this, they don't know what they are fighting against. And i could easily see him lose interest or motivation.