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As always, it depends what wou want to do with it.
For headshots to full body portraits inside, where you control the light, speedlights are sufficient IMHO. For your intended use of dogs against a black background, you should be set.
I don't understand why the "pro" photo shops you called said speedlights are only good for outside. That statement doesn't make sense in this context. Maybe they meant for inside, a good studio flash head in a large softbox gives the best light, but IMHO you can get pretty far with speed lights inside.
The different story begins when you want to go outside and try to overpower the sun. Here, the more power your flash has, the better. I've shot half-body portraits with just a SB-910 outside, but you have to get really close with the flash, meaning your light will falloff very quickly (inverse square law).
For inside, I think the Godox V860 should be fine, especially when paired with the Canon speedlight so you can work with 2 lights. For a bit more power, I'd consider the AD200. Me, I'd go with the latter, as it has the option to be used bare-bulb, that might fill a large softbox more evenly.
But if you plan to go outside at some point and are willing to spend the $$$, I'd go with the Godox AD600B.