While I've not tried Catalina on the Designare build, I have run a 2nd SSD with Catalina on my X299 build. It was absolutely straight forward: same kexts, drivers and SSDT files. Even sequential updates from ß6 to now ß10 have been seamless. I've used both OpenCore (v050 and 051) and Clover (v5070).
One significant change I noticed with Catalina is that it does not seem to require EC injection for power management (I made a post about it
here).
There are some interesting security changes with Catalina that are nicely described
here and
here with some SIP background
here and
here (this site, The Eclectic Light Company, has some really interesting art history too and I was then off reading about Charlemagne).
On the same site, there's one post that deals with upgrading to Catalina here (of course, not from a Hackintosh perspective, but still useful to read).