I've seen public sector organisations where FOI laws have been used by salesmen spamming to determine technology infrastructure to pitch their product.

It's not why FOI was created or motivated. There is genuine need for openness but this sort of thing just tasks already underfunded and stretched departments with work that distracts from providing core services.

I can see how that would be frustrating and sort of causes a misuse of public funds.

Feels like infrastructure queries shouldn’t come under FOI when i first consider it. Then i suppose there might be legitimate reasons to care; Perhaps things like conflict of interest in contract allocation, or things not being put out to tender.

As you mention, it feels like publicly funded things should be able to be inspected by default, so perhaps having it out in the open all the time is best. That requires funding and it requires a commitment at a governmental level to get it right.

With the lack of funding provided to public institutions in the uk though it’s pretty hard even for the basics to be done well. I’d better not get on to that subject lest i get testy.

Back to your main point though, I don’t know how to differentiate (with a set of unambiguous rules) between the trivial and non-trivial in a way that doesn’t accidentally prevent real use cases.

Perhaps the way FOI is set up is at fault here rather than the people who make use of it, trivially or otherwise.

FOI laws have been created so that the public can gain information about the internal workings of the government - and the scope is deliberately wide. It includes activities that you or me may not consider worthwhile, but that’s our judgment call. If handling FOI requests overloads the departments, then the answer would be to just make the information accessible to the public from the outset or supply sufficient funding to the departments so that they can provide the services required by law, including answering FOI requests.