I've cut my subscriber list to 3,000, but I have the 1,001-2,500 plan at Mailerlite. This allows me to not waste money sending to subscribers who rarely open the newsletters.
However, it's difficult to weed out just the right amount of subscribers so that my mailing goes to almost 2,500 recipients. I typically exclude anyone who's been inactive for x days. I play around with x to get close to 2,500, but it's tricky because you might be sending to 2,400 subscribers, but use 2,600 unique subscribers in the last thirty days. You have to have tried this to understand.
You can't just tell ML to send to the first 2,500 unique subscribers.
In any case, has anyone dealt with this issue and found a good way to deal with it.