a mining pool does not need to self fund billions of hardware and electric to achieve managing dominance of network hashrate. they can just incentivise independent asic owners to join their pool.
(imagine a government managed pool offering asic owners tax free earnings or other tax rebates/grants/incentives/public services)
(imagine a electric company managed pool offering asic owners free/discounted electric or other incentives)
(imagine a CEX/merchant offering zero fee trading/withdrawals on all reward deposits)
[tin foil hat] ban mining in a state/country unless they use government official pool
[other examples exist]

so a majority hash attack need not be expensive for the mining pool manager..
infact just being a mining pools actually has low operation costs as it is now

the hopes of avoiding this strategy is that the independent asic owners notice the malicious mining pool is causing annoyances and they jump out of the pool and join honourable pools

anyways, pools cannot change the rules by just sending a funky format block.
but here is a list of some(not all) things a network majority hashrate mining pool could achieve:
       - ignore transactions of certain government black list/sanction list of known funds from certain countries/entities
       - ignore transactions of certain government black list/sanction list of known funds to certain countries/entities
       - ignore transactions of certain known funds to certain unregulated, unregistered services
       - empty block (no transactions at all) just to annoy the network, cause mempool congestion
       - ignore low fee transactions but only include extreme high fee's
       - ignore lean transactions but include extreme bloated transactions
       - ignore all transactions but include their own bloat, spam transactions with high fee, at no real fee cost as fee comes back to them
       - re-org latest blocks by going back to previous blockheight and re make old height blocks with different/less/no transactions included
       - build faster list of solved blocks but only release 2 at a time if competing pool built 1 block ontop of theres, to remove the competitor block