Maybe a dressage or flat work lessons to add some new tricks to your “flat rides” :)
work on transitions, lengthening and shorting the stride in trot and canter. i also recommend working with poles and maybe spirals at trot and canter, start on a bigger circle, gradually moving in then spiral back out while staying on a circle
If you are able to do poles in your flat work there are many different exercises out there. One of my favourites is raised trot poles but every other one isn’t raised. (I forget what there called) it really helps to engage the horse’s hind end and make them think about what to do.
I recommend doing some transition work to really get your horse under you. For example, everytime I work my horse I do walk to trot and trot to canter transitions. While doing these transitions, it is very important to keep your horse in front of the leg and their neck round. Also, incorporate some counter canter and some transitions that changes the pace. For example, lengthen your canter, then shorten it using your seat and some half halts. Hope this helps answer your question!