Here are some references about building calluses. IMO, few of the suggestions are more than anecdotal other than the key recommendation, i.e., play guitar. There is no other choice... if one wants to build calluses for guitar playing, one must play guitar. Still, no one should fight through 'unbearable pain.' Play in moderation; play more, shortened sessions if necessary, but the bottom line is to build calluses ya' gotta put fingers on the strings.

Some of the stuff listed below may help. YMMV.

Guitar calluses: how to toughen your fingertips for guitar https://guitaradvise.com/guitarcalluses/#:~:text=Eric%20Clapton%20himself%20advises%20for%20using%20isopropyl%20alcohol%2C,that%20would%20otherwise%20take%20a%20month%20to%20develop.

How to build guitar calluses - 5 quick & effective steps https://zinginstruments.com/how-to-build-guitar-calluses/

The secrets to building the perfect calluses https://www.guitarmasterclass.net/guitar_forum/index.php?showtopic=40260

How to toughen fingertips

https://healthyliving.azcentral.com/how-to-get-rid-of-black-toenails-12392861.html