Last year I flew from IAS, 8am flight, left Uman at 8:15pm and arrived at 5am to IAS. Went through mogilov border by car and that was a big mistake as it took 3 hours with only about 15 cars in front of me. The people that crossed by foot at the same time got through in under 30 minutes.The Moldova-Romania border is a joke and took 10 minutes (can only cross by car). Similar experiences when I traveled in middle of the year.
Palanka border wouldn't make sense if you're going to IAS as it's a lot further, I've done it when I flew from KIV though, the travel time is much quicker than mogilov if going to KIV.
I would be very comfortable taking a 9:25am flight if you leave Uman right after the zman and cross the border by foot.