I have a friend using a homebrew "Wizard of Legend" subclass he found online.  I took it upon myself to try to recreate it as best as I can and found I had to take a few shortcuts while creating it, namely only allowing the celestial affinity for those of you familiar with the document I am referring to.  at 18th level this gives the player the ability to choose any cleric or paladin spells as a sorcerer spell (the subclass is originally based off of the sorcerer rather then the wizard as the name may imply).  I am at my wits end trying to enable a feature like this so my player can choose his spells for himself from this expanded list of spells.  Unfortunately for now I just have his choices baked in as specific spells I had to call out and force to have always prepared.  Is there any way I am missing to allow him to choose the spells on his end rather than me having to edit the subclass if he chooses to go with different spells?

As part of the 18th level feature you can create a custom spell list (just like Ravnica Backgrounds and Spells of the Mark) and then add every Paladin and Cleric spell that doesn’t to appear on the Sorcerer spell list. Once that is done, they will be able to choose from those just like any other spell.

I hope that helps.