This implies a few things that don’t feel correct to me. First, there are very large very sustainable projects that are fully free/libre (open source), and license changes to source-available tend to drive people towards those ecosystems.
Also, open source does not necessarily mean that others can leech off of your work and not contribute back unless you use a permissive license as you have for the server. AGPL would ensure any commercial adopters have to share their improvements back with you, even for companies who don’t distribute the software but simply host it on their servers.
Clinging to an open core model I think will undercut the project long term. There are very successful projects like https://element.io/ which would never have gained the traction they have while trying to maintain a monopoly on such critical codebases as all the clients.