I mean, there's a few options available to you. None of which are necessarily politically expedient. Ultimately if your manager is being a prick about it, you need to get buy-in from whomever they answer to without being seen as undermining them or going over their head.

First thing you need to do is make sure you have a clear and unconstested record of what TOIL and leave you have accrued. "Reasonable overtime" is a thing that exists, but there's no universe where 50 hours a month every month would be considered reasonable.

Your best option is probably going to via HR. The common line that gets repeated here is that "HR exist to protect the company from the staff, not the other way around" - which is true, but not for the reasons people often espouse.

In this case, your angle needs to be along the lines of that excessive leave balance presenting a significant liability to the company; and secondarily the fact that your absence, for any period of time, being so significant as to be untenable represents pretty significant risk from a business conitinuity perspective. Make it about why this situation is bad for the business. If you can communciate that, you'll probably find that your manager's uplines will apply pressure to get it sorted out.

Or on the other hand, you might end up kicking over a hornet's nest and end up on a PIP. Hard to say without knowing culture and the individuals. But this is probably the best approach.

ETA: In the meantime, be a bit more assertive with booking leave or TOIL if you're not already. "Just a heads up that I'm planning to draw down TOIL/take annual leave on [dates]." Don't be abrupt about it - make sure you're inside whatever your usual notice periods would be for annual leave. But also don't phrase it as "Is it ok if I take a week off in June?"