I’m not expert regarding C++, but if you want to code your own solution is the way to go.
However I strongly suggest you to take a look at the IK Retargeter, since it can manage pretty much most cases.
Regarding getting rid of the AnimBP alltogether, honestly there are usefull stuff there, from handling simple things to most advanced, so I advise you to use it and just see what is the best solution to solve your issue…as mentioned, IK Retargeter is a super usefull solution to finally manage different characters without relying on a an external DCC.