Having a goal is great at creating motivation and achievable marks. So having a set show day might be helpful for you in needing to know what condition and tune-up training he needs.

A great focus point is making sure the horse is set up to be the best possible citizen - because any horse with a solid foundation education will have a better life than one who has 'issues'. Maybe write an honest 'sales ad' that discloses everything about him and use it as a guideline of what to strengthen. How is he with mounting - does he move before, during, after or until you cue him? Tacking up - does he move, wring his tail, pin ears, seeks out the bit? Grooming - does he pick up his feet and hold them for as long as you need, does he lean on you? Can you catch him in a turn out or stall easily? Does he move away and towards pressure? How is he for the Farrier, Vet, Worming, Vaccinations, Trailer? In-hand work - does he trot on cue, can he pick up a leg and move it when you ask, does he halt straight, does he back straight and true? How is he off property or when away from buddies? On the trail can he be lead or behind or in the middle, does he move up and slow down when asked without being worried about what the other horses are doing, can he go alone? How is he with water? Going over bridges, opening gates?

I have found that listening to the Equine Clicker 101 podcast is so helpful with training - it gives new ideas for things to work on and also things to be mindful of with current training. It's a great step-by-step training podcast that teaches you how to do the training session with pauses for you to try it, and troubleshooting possible problems.

Dressage is such an important part to building and maintaining a great foundation - incorporating lessons and/or sessions dedicated to the building blocks is so helpful for you and the horse. I notice such a difference in my work after a dressage lesson, everything benefits from it. Engaging correct posture (the horse and you) and correctly utilizing the body in a balanced manner truly make another work that much better.
If the horse I'm riding is struggling in the trot, going back to a walk only session can really help in finding that balance and correctly utilizing the hind end to get impulsion and set them right. Same thing with canter work - sometimes it takes a few walk/trot only sessions with a fair amount of lateral movements to build up the strength in the hind end to carry the horse and rider successfully into a forward and elevated canter.

For some self-contained lesson ideas - try Ride IQ which is an amazing coaching app.

For some pattern exercises - try Equisense, I like that there's a few pages of interesting set ups and maneuvers to work through for various things. Like doing a four leaf clover circle pattern over poles or around cones to work on bending and balance.

Equilab is a great app for ride tracking - I like that it makes me think about the ride and I use it like a journal, as well as being able to look back over the previous rides and see changes and progression.

Desensitization is always good to work on - having a horse who can be exposed to as much chaotic things as possible and be chill. And training them to want to approach/investigate potentially unknown/scary things is great for when you come across them in the 'wild'.

Try a different discipline - Working Equitation, Mounted Archery, Medieval Games, Sidesaddle, Barrel Racing/Pole Bending/Gymkhana Patterns, Mounted Games, Driving/Ground Driving/Surcingle. Go to a clinic/show or just practice it at home

Sagamore Farms said:

Theres about a million things you can do with a blank canvas like that, and I think Boulder Creek has listed some great ground work points to work on.

That being said, ill offer you three things I look for in a competition horse: confidence, consistency, and lead changes.

Confidence: jump him over anything you can. Fillers, flowers, plants, literally anything you can find. Pop a cone under a vertical, or throw a towel on the fence. Spook him a little so that eventually life is less scary.

Consistency: the basic riding mantras. Can we maintain our pace before and after fences? Can we not cut the corners of an arena? Can we halt in a straight line after a jump/pole? Can we stand still while being ridden? Can we do clean walk to canter transitions? Can we keep our brain screwed on? All important questions to ask your pony every ride.

Lead Changes: Essential. I do believe that the bane of a riders existance is trying to compete on a horse without flying changes. If he doesnt have them yet, he needs them. If he has them, make them auto.

Those are the things that in my mind make a proper show horse. They dont have to be able to jump high to be a solid creature.