You can use a custom origin certificate, but that’s irrelevant when CloudFlare still re-encrypt everything to analyse the request in more detail. It does leave me torn when using it, I don’t use it on anything where sensitive plain text is flying around, especially authentication data (which is annoying when that’s the most valuable place to have the protection), but I do have it on my matrix homeserver as anything remotely important is E2EE anyway so there’s little they can gain, and with the amount of requests it gets some level of mitigation is desirable