The first thing I would tell you is the main things which hurt your domain deliverability are too many unsubscribes or spam reports. If you're not getting those, then I wouldn't worry too much about your domain deliverability.

I would suggest coming up with something "free" to offer like an small eBook or guide. You can also run a contest and give something away for free to a subscriber. By offering something for free that has a high value, you can entice those people who may not respond otherwise.

From there I would continue to send your regular emails for 90 days to determine who opens/engages. After 90 days, I would send an email asking those who didn't open any of those emails if they want to stay on your list. Some email programs make this easy to track by allowing you to tag subscribers who click on a link. I use ConvertKit and they do this. Otherwise, you will have to ask them to reply back or maybe click on a special link. Wait at least 3 days before you delete those emails which haven't opened.

You will end up with a smaller list, but it should be a much more engaged list which will give you a much better ROI in the long run.