It would be nice to see more spell customization options for existing D&D spells, such as being able to change the Range/Area, Damage Type, or Casting Time of a spell.

You can if you homebrew a new version. How do you mean?

What I’m specifically referencing are spells granted by abilities. For example if I create a way of the sun soul monk and want to change the level 6 ability Searing Arc Strike to a 5ft wide line with a 30ft range rather than a 15ft cone with a 15ft range, I do not have the option to do that. I also cannot change the type of damage say from radiant to force. So yes I can home brew spells, but I do not have the option to influence spells granted by class abilities.

Like how grave domain makes spare the dying a ranged spell? Yeah the only tool features have for changing spells is range (specifically because of that subclass). The homebrew tools are the same tools the devs put in official options, they haven't been made to do extra stuff that has never been needed (though they are planned to be rebuilt with more creativity in mind).

Quote from Plasmapaint >>

What I’m specifically referencing are spells granted by abilities. For example if I create a way of the sun soul monk and want to change the level 6 ability Searing Arc Strike to a 5ft wide line with a 30ft range rather than a 15ft cone with a 15ft range, I do not have the option to do that. I also cannot change the type of damage say from radiant to force. So yes I can home brew spells, but I do not have the option to influence spells granted by class abilities.

 
Have you considered creating a copy of the subclass and editing the modifiers there?