Riding vest and reflecting breast collar for the horse.
The le mieux ones are super reflective and good quality
a bright saddle pad and polos. maybe a vest. many people sell vests used for riding in the roads so cars and stuff see you. Also you can find bright and reflective gear used for riding in woods or trails to avoid hunters
A high Vis neckrope is also quite ideal, it means you have a lighter object instead of a big jacket
I know horze.com sells some reflective gear for around $20. They have a harness, halter, bell boots, and etc.
A vest you could wear, maybe a helmet cover, some horse boots, and if you could find a saddle pad that would be really cool!
Wear a really bright neon pair of polo wraps
Look at don't shoot me horse gear. Great stuff for hi visibility.
I wear a high vis gillet when I go riding on the roads, you can also get high vis exercise rugs for the horse, or reflective stirrup clips or band for your helmet
I would recommend buying reflective boots for your horse, a reflective browband, and a reflective decal for your helmet. If you want to be extra safe, you can also use a bright coloured saddle pad and jacket for yourself.
Hope this helps! :)