I didn’t actually look last time but I’m not seeing a critical.css or critical_min.css file being served. I see what is likely a malformed attempt at it. Check how to use link tags at https://www.w3schools.com/CSS/css_howto.asp.
As for the JS loader, you should be either starting it immediately (without the current wait) or get rid of it. It kinda just gets in the way otherwise.
To your more general question, FOUC is bad and looks very unprofessional. FOUT (flash of unstyled text) can be okay if it’s not too jarring – I do that on some sites while fonts are loading.