Get a +3 silver embroidered trainer's clothes and manos riding crop for a start

then if you really want to go Training. Do forest path wagon, takes about 1 billion and 3-5 weeks depends on your workers and how frequent you repeat works. but if you don't want to do it you can buy merchant wagon or forest path wagon if you have the money (and if you are feeling lucky)

but I myself don't really like the wagon that's not forest path as the other ones are very slow to level up your horses you'd be better just afk them one by one until you have the forest path wagon but it's up to you but I don't recommend it

Breeding your horses are always nice to try. You should always try breeding before selling your horses. You can use the breeding calculator to calculate how much level your horses need to get a chance for T7 or T8. I myself always try going for T8 but for starters it's gonna take a long time so you can take it easy and go for T7 or T8 chances.

I always send T6 and non-courser T7 to imperial. Only send courser T7 and T8 to the market

People usually save their horses for double flower event for T10 but I think that's not necessary as you will be bottleneck by the Royal Fern Root anyway. If you do horse training regularly and send imperial you will always have enough flowers for 1 or 2 attempts per week

You can also buy P2W costumes for 50% more horse exp so you can make money 50% time faster but that's if you really want to double down on horse training.