So if i had to do it i would need to clean install right?Well, I finally reinstalled it. Neither factory reset, nor recovery change fixed it, I also completely removed Magisk. At the end I had to use Moto Rescue Tool to install the factory image, then did a clean LOS install. Now the OTA works fine.
So it seems the in-place LOS18 > LOS19 update didn't work too well, at least for me.