Steve,

Since hashtags cant really do much when they are in an email subject
line,

That's the only place they do anything, in Groups.io.

After all, they are not relevent at an email level... at least with my
group they are not.

Oh, but they might be, to any of your group members. See the following:

Another option could be to let the user subscribe to only certain
subject tags and not others.

That part exists. See the description of *Following Only* in the help:
https://groups.io/static/help#suboptions

Or look on your Subscription page, in the Advanced Preferences section.

Shal