It depends a little on your intended market.
I don’t think the ARIA certification will have much impact to dressage riders. I think it would be more meaningful to moms with kids, or to other beginner sources worried about safety.
The USDF certification says to me that you’re serious about your teaching, though it isn’t as huge a positive as I wish it was. (I am much more positive about the USEA instructor certification.) It’s also a good networking opportunity, and would get you in some directories, I seem to recall.
A Bronze or Silver medal tells me you’re serious about your competitive riding, and it tells me you have some experience and some ability to put things together. Yes, schoolmasters can be leased… but you’ll learn from the experience, and you’ll earn those scores. We don’t call 'em schoolmasters for nothing.
Regular clinics with a top rider improve your skills, give you some networking, and may help you get better scores with your current mounts and levels.
Then there’s also just going to more shows and/or placing ads in local magazines and directories.
It’s worth considering whether you’re someone with school horses and who is able to work with raw beginners, or if you’re someone looking for people who are already going and doing dressage on their own horses. These are quite different markets and will respond to different approaches.