Not really, and not as much. Not really because one cannot begin using their own changes unless and until the upstreaming process concludes successfully. Not as much because, unlike with open-source licenses, one does not get to keep their copyright.
One of the important reasons I personally use and support open-source is the freedom to not only inspect (which the TSL provides) but to also not have to ask someone else and wait on them to make any changes I need to the software I use. The restriction against production use prevents that.
One of the important reasons I personally don't mind contributing to open-source, is the fact that I get to retain my rights.
> This is only for the parts of the code that are licensed under the Timescale license (most code is not).
This is a moot point because without the parts that are TSL licensed, we'd not be having this discussion.