I'm not sure if the modding api allows this, but I would really like it if the schedule of many trains could be modified at once. (specifically changing all similar schedules)

Hmm, basically a checkbox to make it do a global search and replace instead? It'd likely be quite UPS heavy as it'll have to scan every trains schedule individually and I'm not sure if it'd play well with LTN etc, but it's certainly technically possible.

I'll have a look over the next few weeks or so when I get chance

For inspiration, you could look at how I fixed that in my mod: https://mods.factorio.com/mod/TrainRoutes
That mod does not have UI-elements, so your mod may make the bulk-changes much more convenient for the player.