1. You are going to have at least 18 months to prepare for an upgrade. That’s going to be 300 nights in a hotel room (give or take) when you could be studying for it.
2. That’s 1000 hours with likely at least 700 hours above 10,000 feet where you can be pumping your CA for what you need to know for the upgrade rather than talking about sports or which FA is the hottest.
3. And yeah, you COULD go to another company although that might require going to a different airframe (and certainly another training event) and - even if not - the standards there might be different even for the same aircraft but just close enough to your old company to foul you up.
4. And seriously, do you want to explain when you are interviewing at a major (or even another regional because they WILL ask) that the reason you quit your first company as you approached 1000 hours was that you were unsure you would be competent to upgrade and didn’t want to risk a black mark? That’s really not the TMAATT you want to impress them with.