transitioning to formula

I have been pumping since 4 weeks old due to tongue tie/ difficulty latching after correcting. My supply was slowly dwindling each time I got my period the last few months so I've been scaling down the pumping over the last month...also for my own sanity, was getting a bit much 5-6x/ day. After 6.5 months the pumping is about to come to an end. I have two freezers of frozen milk I've begun to use but looking to know how you guys transitioned to formula...did you mix with breastmilk and if so for how long? Also, what brand/type did you use and how did your babies handle the change?