I’m running some laboratory testing in a mixed acid environment and would like to use a corrosion resistant polymer as apposed to a metal. I’ve tried PEEK screws but am still struggling a bit with some acids. Is there anything better?

PEEK screws provide reasonable corrosion resistance to most acids but is more geared for high strength applications in moderately corrosive applications. For a polymer fastener that is more corrosion-focused, I would go with PTFE fasteners. PTFE screws will not provide the strength of PEEK, but they are much more corrosion resistant. If the strength of PTFE screws are good enough (approximately 3900psi at room temperature) for your application then you likely improve the corrosion resistance of your fasteners.