I don't think you need to go out of your way to prop up a libre relay network. If you run knots, there's no difference to you whether a spammer uses core or Mara pool. Its gonna be negligible on fee rates (and thus miner centralization) unless theres more spam as a percentage of transactions, than miners willing to mine them as a percentage of global hashpower.
Mara, in running a private mempool, is gonna have to take personal responsibility for what they mine, and their gains from mining spam will probably be outweighed by losses from filtering out non OFAC compliant TXs.