Hello! Listen to your self:
I've built a board and been using it pretty regularly
I've since noticed a dull ache in my middle fingers of both hands after being on the wall
I've injured these fingers previously
I don't think it's anything particularly to worry about
Of course it is something to worry about! Steep increase in volume and difficulty (assumption) combined with insufficient warming up? Injury is on its way.
I have some massive jugs on my board made from 3"x3" wood that are super for
training warming up on. If i keep my feet on the ground it is pretty simple to spend 5 minutes using the holds with varying amounts of weight taken by the feet to start getting the blood flowing etc. I then move to short links on the jugs and then longer ones, with varying rests between.
Also I use a pull up bar to get things going sometimes before going on the board.
Sometimes I start warming up and a finger is sligthy achy from a previous session so I stop and do something else. Often the next evening it is fine to train on again.
Think about icing after every session also. I think 10 mins with your fingers sat in a bowl of ice water post session will make a big difference to your injury proofness.
Above all take it easy, you will get maximum benefit from your board if you are still training on it in 4 months - 1yr, not if you go mental now and then spend 2 months rehabing a cod finger.
Or maybe it is just me that gets all keen and then gets all injured and then gets all keen again and then gets injured and then gets keen and then blah blah