That all sounds absolutely brilliant. You are so lucky to have such an area.
I haven’t ever been involved in that sort of thing, but, for what it’s worth …..
It sounds as if you have done a fair bit of money spending already with the hog houses and the leaflet drop. Most of the things I can think of otherwise, by way of improving the habitat, could probably, as you say, be done fairly cheaply with the help of volunteers.
I don’t know how much support you have, but it seems to me that education is an important part of this sort of project. So that maybe you could do talks, etc. and maybe if you had some camera traps, you could illustrate to people the progress that was being made. Of course there is the security angle to think about, i.e. would cameras potentially be a target for theft, etc. but something you might like to think about. The more people know about hogs, the more likely they are to help them, so that would, hopefully, ultimately help the hogs in general. At the same time it may encourage more people to become involved with your charity and enjoy the fresh air and emotional wellbeing that a connection with nature can provide.
You may already know that there are lots of downloadable resources https://www.zooniverse.org/projects/zooniverse/snapshot-serengeti/talk/subjects/34794060
But I wonder whether this is the sort of thing which PTES Hedgehog Officer might have some more suggestions about. Contact details firstname.lastname@example.org
Good luck I hope you come up with some suitable ideas and manage to get the grant. Let us know how things go.