These are the sorts of issues that anyone who is considering taking the Bar course and aiming for pupillage should be considering. Whilst I do appreciate the additional factor of you being an internal student, fundamentally there's nothing unusual in this sort of situation. The one thing I would say is that the BPTC isn't really valuable in and of itself. Whilst its vocational approach is very different to a law degree, it doesn't suddenly make you a good advocate or a good barrister. There is a decent gap again between the Bar course and actually practising. So I wouldn't do it for the sake of learning what it teaches. It needs to be a stepping stone either to pupillage or, in your case, career progression back in your home country.
If you intend to stay here that means you just have to critically assess your prospects of securing pupillage, which again is something that everyone in your position should be doing. Getting a job in the interim shouldn't be overly difficult if you're a strong candidate. Plenty of candidates become paralegals or secure other advocacy roles. Getting pupillage before you end the BPTC or within a year or two of finishing it can't be guaranteed and may be difficult, but securing other work in the mean time shouldn't be, and indeed for a lot of candidates getting that alternative role and gaining practical legal experience is an important part of improving their application and ultimately securing pupillage.
So I can't so much offer direct words of wisdom other than to say that if you want to be a barrister and have the sort of profile and experience where securing pupillage is realistic, there's no reason why you shouldn't push on and do the BPTC.