Hello. Not sure which would be easier. I don't have any experience with spring boot and limited Kotlin knowledge, so not sure how much would need to change. It's most likely just the part related to fetching and caching features, which I believe is the only thing relying on Android-specific APIs.
We also have pretty good docs for starting from scratch -
https://docs.growthbook.io/lib/build-your-own
It seems to typically take 1-3 weeks to complete an SDK from scratch