Yes, I have but using a Pixel 4 XL. Candidly, for me, it was an experiment to see if it would work, which with some quirks it did.
The fact a Pixel 5a is capable of using more than one SIM (a physical SIM and an eSIM) simultaneously does not change the fact Google’s Pixel 5a is not currently compatible with Republic service. If/when Republic is ready to say something regarding support for Google’s Pixel 5a, it would be said here: Announcements & News - Member Community.
Meanwhile an option would be to activate the Pixel 5a with Google Fi. If you wish to use your current Republic number with Google Fi, you would need to transfer your number there. Or, if willing to do so, you could get a new number with Google Fi and continue to use your Republic number on your current Republic phone.
If you choose to transfer your Republic number to Google Fi, doing so would cancel your Republic service. Like Republic, Google Fi is a prepaid no-contract service provider, so should Republic decide to support Google’s Pixel 5a in the future, you would be able to return to Republic by transferring your number back. It’s possible you would need to acquire a new Republic SIM to come back.