I generally recommend internships for those wanting to go into ER full time, but I also I agree with what Jayna and Mix have said. If I was set on going into GP, I would not have done my internship.
The level of cases you'll see at a specialty hospital are often (and supposed to be) out of the scope of what a GP has resources to handle. That's why 24 hour ERs and specialists exist. At the end of my intern year, I scrubbed in on who knows how many septic abdomens, cholies, nephrectomies, etc. Watched hundreds of echos, assisted during many phacos, and so on. If I were going into GP, probably 80+% of what I did that year would not have substantially contributed to the skills I'd need for GP, and the biggest learning point would have been when/what to refer. I think that is valuable knowledge, but would not be worth the lost pay/time to many. Given what you've stated here, it would be far more beneficial for you to go into a really mentorship-heavy GP clinic and take surgical/ultrasound/ER/dentistry/other CE to get you standing on your own two feet ASAP (since you want to open your own clinic quickly). You don't need to do an internship to learn how to tap a pericardium/chest, GDV, splenectomy/hemoabdomen, etc, which imo are some of the surgical/ER skills that would be helpful for a GP.
If you are dead set, though, I did my rotating internship at BluePearl Tampa and I’d recommend it. Biggest drawback is that you would not be primary surgeon on anything, which is pretty important for your stated career goals.
I am not sure what you mean here. If you are thinking your intern year will leave you with the ability to manage very complicated cases in the setting of a (brand new) GP clinic, I'd disagree for the most part. Just because you know how to do it (and I feel most GP vets can do anything) doesn't mean you'll have the staffing, time or equipment to do it. Perhaps our definitions of what constitutes a 'complicated case' are different.