Wow Linda, you have done so much in your recruiting efforts!
The only things I can add:
1) Presenting to conversion classes - in our experience in Chicago, 'new Jews' find chevra volunteering to be something they feel comfortable doing. Of course, having the rabbi or teacher of the class promoting the concept helps a lot!
2) Finding one or 2 millennials or Gen X-ers to engage and promote it within their demographic circles, like during a Shavuot tikkun or High Holidays (my daughter Leah, a chevra member, taught at her chavurah's last tikkun)
4) Presenting to the local Avodah alumni network (my daughter is an Avodah alum and just did a CK training here in Chicago. She could probably do one on Zoom for Boston folks if there's not a Boston person to do it)
5) Cosponsoring a conversation about Jewish end-of-life with Shomer Collective or Death Over Dinner people
6) Continuing to present annually at several of the places you have been.
7) Are there more folks from the Boston Community Hevra who live nearby where you do your taharas, who would be willing to join both?
8) In Chicago we have 6 congregations who actually are members of our community chevra, and reps from each one are responsible for engaging with their own community and bringing new people in. That way it is a known,familiar face teaching, not an outsider.....
I don't know if any of these will be helpful! Good luck!
Take care,
Liz