3) is a bad idea since you should expect a DB engine to lock access to a single process by default. I would not base my code on this assumption, which imply you would need to bootstrap your own history DB without Core running, then use a different code path to maintain your own DB straight from blockdata.

2) is tedious and what are the chances that would be merged into Core?

1) is how I would do it, pull all blocks and check each for relevant UTXOs. This process can be very fast if you parallelize it, but you don't necessarily need to since it's just the original bootstrapping that will be resource intensive. Maintenance won't be nearly as costly and can use the exact same code path with bounds on block height.