sounds like you are doing a great job
First off I would advise you to buy a book by Jaime Jackson called paddock paradise. This book is based on feral horses and how they use their home ranges. There are a fair few people here who have this system set up but I cant remember who exactly
I had a track system at my last place and it works fantastically well.
The basic idea is that rather than just plonking the horse in the field, there is a track that goes around the field (or through it, across it etc.) that is infinite. In other words imagine a 2 acre field with a track going all the way round so the horses can walk for ever! The track has to have a surface, often rolled hardcore, but also some different surfaces for conditioning and interest.
Along the track are placed various things e.g. the water trough, a larger sandy area for rolling, salt licks, ad lib hay all along the track etc. etc. really you can use your imagination, and this encourages the horses to move. The horses are then let into the centre of the paddock to graze for various hours a day (or whatever works). The adlib hay on the track is extemely important as horses need to eat for an average of 16 hours a day.
This solves various problems
1. Lack of movement. Most horses do not move anywhere near enough
2. Helps Conditionin the hooves necessary for successful barefoot horses
3. No excess of grass as horses are not designed to eat such rich food
(especially good for lamanitics) but ad lib hay
4. Enables herd life
5. Different things to do on the track aid enrichment
I do not stable my horses as this inhibits all of the above but obviously the answer is not to just have them living in a muddy field. The track can be expensive to set up but can be done little by little. Meanwhile, if there is anyway that you could make a small yard area outside the stables and give them free choice to come in and out. Many of my clients have removed the partitions and doors of the stables. It is important that the horses get on and dont get trapped by a horse that maybe has issues and that the introduction of a new horse is done properly. But thats another story!!!!!
Hope thats given you some food for thought and I highly recommend the book (although there are a few bits I dont agree with
) but thats a basic overview.
All the best