We see this a lot - People want performance of modern CPUs but don't trust the platform. That's totally understandable but is sadly unavoidable in the x86 world. Even though coreboot easily provides the most transparency you'll get in x86 host firmware these days, it's still just one component that you need to worry about. ME cleaner is great but has obvious limitations to what it can remove.

If you're really serious then you need to divorce x86 and try an alternative architecture such as POWER9. Raptor (https://www.raptorcs.com/) sells reasonably affordable systems with Xeon-class performance, PCIe Gen 4.0, and 100% open source firmware for both host and BMC. Freedom isn't free, though, and if that's too expensive you might find a cheap used POWER8 system (not as open as POWER9, but still good compared to x86 systems in terms of openness): https://www.phoronix.com/scan.php?page=article&item=tyan-power8-server&num=1.