How can I create fun yet fair challenges for a solo Sorcerer campaign?

Playing in 5.5e (2024)

After my last post gave me confidence and ideas about how to DM a solo adventure for my wife, we have started to talk about what kind of game she would like to play.

She has decided to play an elven sorcerer on a journey of magical self discovery. But as a full caster, I'm slightly concerned about how best to craft encounters that won't be too difficult and unfair. Of course I plan to craft various DMPCs to join and aid her, but she says that occasionally she would like to journey and fight alone. We created a character on DnD beyond who has a good balance of attack spells and plenty of constitution thanks to a farm background that gives her a starting HP of 10.

Anybody with similar experience, or sound advice, how would you approach writing various encounters for this campaign? Any suggestions are gladly welcome 😊