Fetch all followers at TTS Application boot, update internal followers list via webhooks.
or
When someone says something in chat for the first time that day, fetch their follow status.
That is the purpose of webhooks. Collect new information in near real time (delay to make sure the data is correct/updated in helix similar to long poll aka caching)
That only works if you have been working with that streamer since they got a sub button, if suddenly I start working with Ninja tomorrow, I don’t have the cached data to form a cached collection of streaks to do this math or comparison on.
If you are using TTS on a channel with millions of followers, and thus logically even 1000 of them turn up to a stream, your TTS will never catch up. If it’s just reading out what followers say in chat. Lo behold the day two followers write a wall of text one after the other and now your TTS reader is 5/10/15 messages behind…
Can you perhaps expand on your use case to describe what is triggering the TTS?
Edit: also this excludes the probability that a large streamer is more likely to have Followers only mode on in chat. Which makes the whole follow check redundant