Throwing some random ideas here regarding the free DA problem, unsure about extra complexity involved but there may be some merit:
a new EIP2718 transaction type specifically for transactions who want to make it into the inclusion list (possibly with a bribe for the validator who puts it in the list) which forces the user to pay max fee × gas limit ahead of time when it makes it in the list, rather than when it’s actually included in a block. The only possible refund is for unused gas if/when the transaction makes it in a block. We can possibly force some rule like the transaction’s max fee must be greater than 10x the current block’s base fee to have a more-than-reasonable shot at being included in the next few blocks
alternatively, the inclusion list could merely contain transaction hashes - with a multi round PBS, we could have a scheme where block builders have to sign a message ahead of time saying “yes, I have seen the contents of the transaction with hash 0xabc... and will be including it in my block” in order to be considered as possible (this may open up new attack vectors with proposer/builder collusion w.r.t. private order flows, maybe we need strong commitment from validator committees to be reasonably confident that the transaction has in fact been seen by enough people for block building to remain open and competitive)