I’m pretty sure you can already do that through the xda developer forums, providing there’s a maintener for that specific phone. When they publish their official or unofficial ROM, they usually provide information on how to donate to them.
I’m happy with the efforts of /e/ to bring their OS to as many phones as possible but it’s really tough to keep up with that many phones’ security updates and patches, and there’s more to consider than just that but also support from the manufacurer, security updates mainstream Android, and also of course LineageOS itself.
If you want to contribute for a specific model then I’d take a look first at Lineage and XDA directly first.