Aight, I found my solution:
This won’t work for people that additionally put default tags on their Meshes, but I just check if the selected Actor has Tags. If it doesn’t, I add its Default Materials as Tags to it. If it already has Tags, I check if any of them match the Material Name I’m specifying. If any do, I can just take the Tag Index (which matches the Material Index) and replace the Material in said Index.
that said, I just noticed that this won’t work in Cases where 2 Slots contain the same Material. I’ll quickly fix that, but that’ll be simple enough so that I won’t update this post anymore.
I hope this helped anyone who wanted to do similar things :>