If on a budget I’d say just invest in a carbon soled MTB shoe and keep your current pedals. I don’t really agree with anc above, I’ve used the same Spesh insoles in s-works road shoes (on Speedplays), BG Expert road shoes (on Speedplays), BG sport road shoes (on Speedplays), Sidi Dragon 2 MTB shoes (on XT SPDs) and BG Sport MTB shoes on XT pedals. I had noticeable hot-spots on a long MTB ride with the BG Sport MTB + SPD combo and the occasional hot spots with the BG Sport road shoes on Speedplay combo. They are the only shoes I have with a non-carbon sole.
No doubt though a carbon sole isn’t the whole story, I agree insole, pedal type, cleat position and shoe fit all come into it as well. Still, based on my experience, I’d try a carbon soled MTB shoe first if I were the OP.