I haven’t reread the whole thread, but I just had a quick thought skimming AJ’s comment: if a large transaction is pre-signed and low feerate, limiting it to a cluster of one would mean that it cannot be CPFPed. So I was wondering whether it would be reasonable to limit it to a cluster of two transactions instead, where the second transaction is similarly limited as the child in TRUC transactions. Otherwise, the only way to get a low feerate pre-signed large transaction mined would be an out-of-band payment.