So since RenBTC is not a real bridge and WBTC is centralized due to BitGo, the only safe way to do this would be to immediately sell the wBTC for ETH and hold that on the ETH chain? Or some other stablecoin?
It depends on your wishes you should have a reason before doing it.
Both are good in their own use cases, for example you want to gain access to ETH "defi" ecosystem or something like that. Converting BTC to ETH/stable coins just to hold doesnt have many prominent advantages.
Plus, there is really no reason for it anyhow. I get that some people may want that but that doesn't mean that wanting something equals to logical thing. You may want something that is illogical and useless as well.
You should not really need or require an onchain ETH type of wrapped bitcoin on that chain, it is really not a smart thing to require, just get out and get bitcoin and it is equally possible. I get that some people do not want to pay the transaction fee but risking it that much gives you bigger risk and the transaction fee looks a lot less costly at that point. Either you lose 5 bucks on the fee, or you lose it all .If you are not aiming at just BTC and want to stay in the chain, pick something better.