Looking for a 3rd party Mailing List module replacement

I’m looking for a 3rd party add-on that will fully replace the 1st party Mailing List module that was removed from ExpressionEngine with the release of EE3, so that I can finally upgrade a client’s website to use EE5/EE6.

The client runs a small non-profit advocacy website that needs to send an average of 2 newsletters per month to a small fluctuating list averaging 50 subscribers. They’ve been doing it successfully for about 7 years using EE2’s 1st party Mailing List module and the web host’s standard email system.

Everything I can find seems to require using a 3rd party marketing email system such as MailChimp, Campaign Monitor, etc. These systems either have a fee attached or force their branding on the free plans. They’re complete overkill. The client doesn’t need/want the complexity of an email marketing system, nor do they have the budget for it.

We want a solution that continues to fully automate the subscribe/unsubscribe process, so that the client can simply log into the EE control panel, compose their text-only newsletter and send it, just as they do now in EE2.

I tried building an add-on, but I don’t have the skill for it, and I no longer have the time to try again.

Any recommendations?