Can I ask what is the purpose of this?
i don't know how does bsgs work but i think it the same core idea.
A range of 1 billion can only solve an up to maybe a 59-bits private key of a known public key, if this is what you are trying to do. But I think you got the strategy wrong - the target point needs to get subtracted in sqrt(rangeSize) delta increments and then looked up in your "database" (which should actually be used with a bloom filter first, and definitely not as 65536 text files).
we are not subtracting the target we are subtracting from the target. Ps(Strat range) Pt(traget) pb(point of scalar 1billion).
PS +Pb =Q , Pt + (-Q)=R , then we look for x of R in Db .
What do you mean by "the target point needs to get subtracted in sqrt(rangeSize) delta increments"