That's a tough decision. I can relate to the feeling of being dangled on a string of uncertainty, never knowing for sure whether she will turn up that week/day or not. Whereas if you start in Sept, you have the certainty - or higher probability at least, that she may be back.

You said you don't really want to explore other Ts, fair enough. If I were in your shoes, despite the uncertainty (which I hate!) and/or subsequent disappointment if she didn't turn up, I would still keep my appointments with her. Though frankly, I think I would have looked for additional support in the interim i.e. while my T was out.

But that's just me. I guess you could weigh both options and/or which is 'worst' - i.e. would you be able to handle making appointments and having her not turn up vs. have to wait until September (with a higher probability she might be in but then again, with a slight possibility she may not)? Do you need the support right now? Would you be able to get support in case something happened / you needed it? IF you don't really need therapy or if the juggling with 'will she be here? will she not?' is too much, then maybe taking a break would indeed be better... or at least offer (some) peace of mind.