Yeah, that’s what I expected. I would have been very surprised if removing FRP was possible with microG.
I don’t see another way out of this one, sorry 
There are a few tools out there that claim FRP bypass, but mostly with phones from larger brands, older Android versions etc. …
Ultimately you still would probably need Play Services to clear it, the necessary mechanism doesn’t seem to be available from microG, and without access to fastboot flash there’s not really a lot of options left.
Honestly surprised you’re the first person I see running into this issue, not flashing FRP seems like a major oversight on the iodé side, there’s no reason to keep it intact.
I also checked the manual installer for /e/OS in the meantime, unless there’s something I’m missing they don’t seem to flash FRP either, apparently my experiences with CalyxOS and a few others were actually an outlier.
I don’t get it, flashing FRP should be default, there’s nothing to be gained from keeping it around, especially since both ROMs don’t ship Play Services, the only thing you’re accomplishing are potential bricks, like in your case…