Are stand alone horse stables operable with the Businesses and Hobbies EP?

I don’t think the “residential” room designation does anything unless you’re on a residential lot. Just like on a residential lot  the “public” room designation doesn’t do anything unless/until you register a business there.

You could try to travel with your horses to your business venue and leave them behind, but they may not stay and will not have anyone caring for them. Small businesses can only open while the owner is on the lot, so if you had planned to hire employees to care for the horses they’ll only do so while you’re already there anyway.

Would you be willing to use cheats? What if you moved the horses into another household with a sim set up as stable master, then when your sim is visiting and wants to compete, you turn testingcheats on and use “Add to household” - or even without cheats, you could go out to manage worlds and just transfer the horse temporarily to your household. Then move them back to the stable household after the race.

PipMenace​ - Thanks for your perspective. A phrase I've been thinking often when receiving input from others recently has been "don't miss my point trying to make yours". 

My sim rents her penthouse, and will also manage the stable regularly whether the business is actively open or not. Given she owns the business venue, there is an option to set rooms on said lot as 'residential' and each horse is a household member, they should technically be able to stay on the lot their assigned beds are on (in this instance, I should even be able to transfer them onto the property permanently). 

Your third paragraph was money. I created a random sim before my post and gave her an adult horse. I could just sell the venue, repurchase it with your mentioned Stable Master and utilize the 'add to household' function when my foals are eventually ready to compete. However, (thinking while I'm typing so I don't know if factual) I don't think I'll be able to train the foals to eat and all the other building skills I've been uncovering since my build. For example, even as their owner Honey is unable to currently ask each foal to eat, and she must individually train them to eat prairie grass.

I'm just brought back to, it makes sense to have such flexibility. This is the closest we'll get to owning multiple properties, so why not make more accessible? It shouldn't be a song and dance to maintain a hobby like championship riding especially when this pack was designed to essentially support thoughtful gameplay (with realistic expectations). 

I actually like intentional gameplay. Even though I toyed with the route of revisiting my build to accommodate a more livable space for people sims, it just defeats the purpose of the experience I'm going for. 

I wouldn't have much to say if it didn't feel like an oversight. The build and thoughtfulness on my end was enough. I just wanna play and enjoy my hard work without too many time consuming extras.