It would make sense to me to replace the part “[, Series 1234: “Subseries Title”][#1234]” with the release date (YYYY-MM-DD), if there is no other identifier.
Even if there is a title that follows (I don’t know if this would be controversial - my experience with podcast files is that being able to file-sort in order of release date is critical).
It seems like the file sorting is a sticking point. I know that there is a preference for supporting the default file naming scheme in Picard, but I have personally switched to “Release Group Series/Release Group Series Number - Release Title” which is far better than brute forcing the issue with prepended dates as it allows mixing single episodes and collections.
There is another lingering issue that I am not quite sure how to handle. Sometimes, episodes are released under multiple series. This is fairly common for BBC radio series which are broadcast as subseries or standalone episodes of “showcase” series like these
I suspect that these are probably better understood as release series and not release group series because they only really pertain to a single broadcast release. However, it’s not clear how these should appear in the titling scheme. For example, this series:
Should this be:
Drama on 4, “Almonds and Raisins”, Episode 1
Or just:
Almonds and Raisins, Episode 1
and should the release be:
Drama on 4, “Almonds and Raisins”
but the release group simply
Almonds and Raisins
because Drama on 4 really only applies to the initial release.
I thought I was echoing your suggestion, to put the date in if there is no other identifier/episode number/etc?
If you disagree with the part where I say to add the date even if a title follows, I think that isn’t a hill I would die on.
I will mention that you might be one of the only people on this earth who has made good use of tagging with MB’s series 
P.S. Do you need scripts to achieve your series-based tagging? If not, it could be a good candidate for a “podcast” preset in Picard.
I don’t currently have time to look at your ‘used in various series’ examples too closely. But it seems that they currently don’t have the program name included in the title, and that seems correct to me.
If they are being broadcast across multiple shows, in different orders, then I think they function more as standalone stories than what I would usually consider a “series”. And I don’t necessarily think the original series they were broadcast within needs to be ‘married’ into the title.
A bit of a tricky situation, though, when it comes to writing guidelines.
Yes, I think we agree on that. I’m not sure about the date plus the title as the date, IMO, is really just the fall back when there is no other identifier.
I will mention that you might be one of the only people on this earth who has made good use of tagging with MB’s series
P.S. Do you need scripts to achieve your series-based tagging? If not, it could be a good candidate for a “podcast” preset in Picard.
It took me a while to realize there was a simple solutions. I use the _releasegroup_series and _releasegroup_seriesnumber which uses the release relationships setting. So I guess its not a strictly “default” experience, but it doesn’t require any plugins.
The one issue though is related to the multiple series for release groups. If a release group is part of multiple series it doesn’t always use the correct series. For example, some of the Prisoner audio dramas use the Big Finish Audiobook series.