Good luck on your link shortener project! I made an open source link shortener many years ago and eventually threw in the towel and quit. The problem domain for link shorteners is one of anti-spam and abuse. Charging for it makes complete sense to try and weed this out but having any amount of "free" I found to be a hotbed of reports thrown against my hosting provider and registrar for supporting spam. I found myself getting quickly blacklisted everywhere since spammers love to use new shorteners for bypassing their own blacklists.

I've not looked into the problem again in 10+ years since. If I did I would 100% skip the free plan.