Regarding your first point, combining 3 API calls into one: Well, it is just one line of code on your server. But you have put the rest of the logic into the Merge/Nango server. Maybe via some point and click interface. But it is still there. So your complexity went up, not down.
Regarding your second point, APIs don't change just nilly willy from /v1/ to /v2/ and let you "fix" it by swapping "1" with "2". Usually you get changes like /v1/weather does not exist anymore and now you got /v2/exact_weather and /v2/broad_weather. Both different to /v1/weather. exact_weather only accepts city names of cities with a population over 1 million, is now a paid service and returns weekly weather instead of daily. And /v2/broad_weather does not accept city names anymore but only coordinates, is still free but locks you out after 10k monthly requests and returns daily weather but without rainfall info.
So a "in between API" won't save you from dabbling with the change from v1 to v2.
And on top of that, now you have two APIs that will change from time to time. The weather API and the Merge/Nango API.
For services Nango supports, they manage all of the things you are talking about - upgrading the APIs, etc. You call weather, you get weather from the APIs you have enabled. They’re selling the management of the complexity you’re talking about.
Regarding your second point, APIs don't change just nilly willy from /v1/ to /v2/ and let you "fix" it by swapping "1" with "2". Usually you get changes like /v1/weather does not exist anymore and now you got /v2/exact_weather and /v2/broad_weather. Both different to /v1/weather. exact_weather only accepts city names of cities with a population over 1 million, is now a paid service and returns weekly weather instead of daily. And /v2/broad_weather does not accept city names anymore but only coordinates, is still free but locks you out after 10k monthly requests and returns daily weather but without rainfall info.
So a "in between API" won't save you from dabbling with the change from v1 to v2.
And on top of that, now you have two APIs that will change from time to time. The weather API and the Merge/Nango API.