Dont know about solidarity.tech.
But i dont like i cant find a clear description of owenership and governance.
I work a bit with zetkin, which i think is very good.
I know its used by jordskred, enhedslisten, vänsterpartiet mfl.. And its GDPR compliant and made in EU (sweden based).
https://zetkin.org/en/
They are friendly if you reach out to them.