Exactly, centralized & controlled is a big issue - but WBTC (Wrapped BTC) on ETH also made a good solution with their custodian system, which I think is an excellent solution.
WBTC is as worse a solution as it gets because not only it is centralized but also it is created and relies upon a centralized and buggy platform.
CoinJoin is effort & also costs fees. I think that my proposed solution is a new solution that does not exist yet.
Your solution also costs fee and possibly even more fees because there are 2 on-chain transactions and also a middle man (you?) in the middle of all this who would require additional commission for the service they provide.
A better solution, in my opinion, would be using Monero itself instead of a new altcoin that is not used for anything else and let anyone join that pool where they exchange bitcoin for XMR and can either hold XMR if they wanted to or exchange it back to bitcoin and get that new bitcoin out from bitcoin pool.
This is something that people already do but use swap services that is not a perfect solution because it is still centralized and the site they use could keep a record.