Or just concurrently sign multiple versions, and the last signer/proposer only completes and announces the one matching the signing set.
The same mechanism can be used to sign for multiple fee rates since it can be useful to fee-bump without having to revisit all the signers.
That said, unless it's necessary for accountability to disclose which signer set signed, you construct a key distribution such that any of the 3-of-5 sets can sign the root directly-- which would minimize fees generally.
On the third point, you are saying to have a "standard" 3 of 5 MuSig1, the only problem is that you are disclosing "all" the members of the group (signers).
As for the first item, it make sense :-) it didn't occur to me, but is doable, the only drawback would be the amount of information I would need to transfer into the PBST (custom), specially on cases with more than 5 participants. As a matter of fact, I would probably need one PSBT per combination fee. Interesting..
Thanks for your thoughts
angelo