Interesting, as I'm not in the key hunting business, I didn't know about https://slipstream.mara.com/ yet. Thanks for pointing me to it. They seem to charge about double the current fee (as of writing this mempool.space indicated 5 sat/vB and Slipstream asks for a minimum fee of 10 sat/vB; I expected worse).

I've read their ToS briefly and it seems at first glance that indeed they claim your submitted transaction remains "confidential" until mined into a block by their own pool.

One can only hope that slipstream.mara.com are really honest and do not exploit the internal disclosure of the public key themselves. I heard that mara.com does attract some critisism, at minimum as far as I recognized for censoring certain transactions which got tagged as "not appropriate". Can't say anything how reliable they are when at minimum 6.6BTC are at stake.