Should I be worried about user names?

The usernames seem to be in almost sequential blocks, as do many of the email addresses.

Definitely something unusual and hence a cause of worry.
Can you link to your site?

Here’s a sample:

Anddausy dx1x4@…net
Anddausz dx1x5@…net
Anddbusb dx1x7@…net
Anddbusc dx1x8@…net
Anddbusd cqbx7@…net

churchthatmoves.com

That takes you to a blank page.

Since when are you getting such subscriber registrations? Any particular date? After installing any particular theme, script, plugins? Can you relate it to any of these?

Seems to have ramped up in the last few days – 31 since yesterday.

No particular event or changes to software (that I’m aware of).

You may close down subscription for some days if you like (or forever). Also review:
https://codex.www.ads-software.com/Hardening_WordPress

Additionally, if your subscription plugin offers it, you’ll want to add in a Captcha

Thanks. Gonna kill it.