Realistically, how does an open source project enforce a trademark, when that carries a high cost (lawyers)?

Said differently, which option (sue for trademark infringement, or for license violation) has a lower cost? (including damages)

If you've released your Software under an OSS License you shouldn't be looking to sue anyone for forking and using your library.

Open WebUI wants the benefits of OSS and "Open" marketing without the freedoms for why people prefer OSS Software.