I am not dismissing anything early. Journalists press law enforcement regularly on what crimes the existing intrusive measures helped to solve and the answers are consistently platitudes about how useful e.g. hacking into suspect's devices is without so much as a referral to a study or even mention of a single example case where it helped. The only data that is available is showing an ever expanding use of these powers. It's nuts.

Checks and balances on police have been tried and the result wasn't even remotely what politicians promised. Judicial oversight, for example, isn't actually working. Police and judges go through the motions, but judges routinely get so many requests for warrants that they sign almost anything in practice because they have too little time to check each one thoroughly. That's how we get police searching the homes of security researchers who dared to report gaping security holes just because they pissed off whoever had the security issues in the first place. It's a pattern that repeated quite a few times. I don't have the time now to dig up a list of cases that followed this pattern and where, in the end, the researchers walked away scot free, but often with considerable financial damages.

Another example: police has pretty extensive databases with private details of individuals. Officers are only allowed to access records in them when their work assignment warrants it. The software keeps audit logs on database accesses so that misuse can be discovered and held in check. But it turned out that nobody ever checks these logs. This came to light initially when police officers started to leak contact details of lawyers to far-right extremists to have them threatened. Under political pressure, the logs were finally reviewed and they found wide spread abuse of the available databases. The result: two years later, the same lawyers and some politicians receive another set of death threats, even after moving to new addresses they kept painstakingly secret. Again, the trace leads back to police officers leaking personal information.

Politicians are also gradually expanding existing police powers in somewhat bad faith. First, it's just for an extremely narrow set of sufficiently horrible crimes like terrorism or sexual abuse of children. And they swear by all that's holy that this will be all that it's ever going to be used for. Then, maybe two or three years later, when public interest is drawn to other issues, the same parties start to steadily and quietly amend this list time and again. [Curiously, many such law changes get discussed and voted on during big football tournaments (think European Championship or World Championship) when they don't draw much attention. But I can't tell whether that's merely the result of election cycles aligning with these tournaments by chance or a deliberate tactic.]

To sum it up: if there is a way to put working checks on police powers, politicians haven't found them or don't establish them in a way that is effective. In light of these patterns, the only realistic stance that remains is to either give police full unchecked access to certain tools or none at all. I wish I could subscribe to your view on this matter, and to be fair, many years ago I would have agreed. But I have been disillusioned since.