I'm running several side projects of my own, and I had hard times to maintain some of them because their domain did not interest me anymore.

Depending on the project and the engagement that you have with your customers, you have several strategies. Either you go in "maintenance mode" with the current customers and welcome new ones without actively doing marketing, or maybe you can sell the business to a competitor or someone interest (there are markeplaces for that), or maybe you can also shutdown the whole thing.

I'd be happy to share my experience with you :-)

Get to know them before trying to sell them anything.
Since you have a large user base, build cohorts of people (let's say groups of 5k people), and send them a fake personal email related to the brand to:
- do they remember the brand?
- why did they sign up in the first place?
Doing this, you'll be able to better understand their interest.

The idea to split the user base is for you to test several approaches. You'll have to use some analytics tool to track these people (email open, engagement etc...).

We can discuss this further if needed.