I would recommend trying an insert too. There's all sorts of different kinds, hopefully you can find something that works. Sounds like a half size smaller would have probably been perfect for you. Leather boots get roomier as they break in and the leather stretches, which is why you need to buy them snug initially, so they'll still fit once they get bigger! Slipping is normal for the first bit until the sole breaks in but if you buy them too big the slipping will just get worse, like what's happening to you.
I used to sell cowboy boots and when customers were trying them on the pair that offered a decent amount of resistance to their foot going in, but then made sort of a THUNK sound as they slid in, that was one of the signs of a proper fit. We learned this from the boot company reps that would come in. And if a customer was inbetween sizes, we recommended to pick the smaller of the two, keeping in mind that you can always stretch leather boots if they remain a tad snug in places once broken in, but you can't make them smaller once they are broken in and the leather has stretched from normal wear. I found cowboy boots and everything behind them incredibly interesting, and fitting them was sort of a science, haha.