Sounds like a good plan. I’ve converted my outdoor ring to grass too. It was never really fancy footing, just rototilled native soil- peat based and well drained sandy soil- I just found it was too cuppy, and dusty (since I can’t get water onto it often- only when our irrigation guns on the hayfield next to it get close enough in their cycle). I added sawdust in a hard spot (not that there was much of a problem with this- just one spot), and dug out a soft spot that was often wet (filled that with sand). I rototilled it, not deep, but just to level it out. It had originally been part of a hayfield, but not really a manicured hayfield, and I’ve never had any actual engineered “footing” brought into it. So just “turf” before I started riding in it, and adding stuff, and rototilling and harrowing. But as I say, it was a bit cuppy. So I seeded it with grass seed. I seeded it in the fall, hoping that things would sprout in the spring when the snow was off. Don’t know how much of it actually did sprout… but the native grass has encroached, and this is very good. “Cooch grass” has a deep mat of roots, and is very resistant to drying out, or traffic. It might die down or get worn, but you can’t kill it, and it will always re-sprout when conditions improve. So, it’s taking a while, but it is spreading, and looking better now. I rode in it a couple times this summer, and that deep pad of roots under the surface was good footing (my horses are barefoot, and it’s usually only me using it). Seemed to have decent traction, and held together under the footfalls.
I did graze it a bit this summer, but my mares decided that they needed a “rolling spot” out there, and dug a spot up for this purpose (very bad girls!!!). So I filled that depression this fall with more sandy topsoil, and I will probably not be letting them out there next summer. I’ve run the brush hog mower over it a couple times this year, and spot treated some weeds (burdock- GRRRRRR!!!). And no, it’s not going to be an international competition grass ring, but is should be OK for me. Hopefully an improvement. Cheap to do.
So, good luck with your seeding and planting plans. Yours will probably be nicer than mine, more level etc. I would not remove the sand, just seed it. Fertilizer works too, if necessary.