I'm doing the same, with
https://httptoolkit.tech/: the core features are free, but there's advanced extras that require an active subscription.
I'm this exact case you cover: it's a powerful developer tool that's very connected to the network & captures the traffic of many of your other apps & devices. It _must_ stay up to date, or users could be at serious risk. Holding back updates from users would be irresponsible. I've noted this in the past, but never really spelled this out fully, and it's important context, thanks!
For cases like ours, the upside of freemium over simple paid software though is that the end of the subscription doesn't completely shut down a user's life (unlike totally losing access to your IDE, or your cloud data), and you do need to ensure that. Instead the free user limits are put back into place, but those should generally make workflows inconvenient or less effective, and not catastrophically blocked.
In my case, for example, free users lose the ability to import/export collected traffic, but the exported HTTP traffic is in a standardized format anyway (HAR: http://www.softwareishard.com/blog/har-12-spec/). Some automated built-in tools go away too, but manual workarounds exist for free, for immediate & short use cases. Those fallbacks & that portability ensure nobody is ever totally locked up by the end of a subscription.
It's a very delicate balance (they need to be limited a bit, or nobody buys anything!) but it's been working well for me so far, and as a bootstrapped dev a free basic option is _incredibly_ useful as free marketing. Good luck with PhotoStructure - feel free to get in touch if you want to chat about any of this.