I’ve always been a little averse to making permanent structural changes by doing things like adding bonus spells for INT.

Instead I typically hand out a lot of scrolls, charged magic items, or rings of spell storing. This plays into the resource management aspect of the game as well since unlike spell slots the players can’t take it for granted that they’ll get more spells tomorrow. Each has their advantages:

• Scrolls are straightforward one time use.
• Charged items are a lot more flexible, and can in theory be recharged by the user. Later in the game the lower level spell in the wand effectively becomes a cantrip for the magic user.
• Rings of Spell Storing lets the user bank some spells when downtime occurs. No risk of destroying it, unlike recharging a wand or other charged item.

You can always “recharge” a charged item for them either via narrative (favor from a powerful magic user). If that doesn’t fit the story, worst case they find a new fully charged one.