Higher level spells provide no benefit, only higher level mobs that are able to be charmed. Unless you're charming mobs higher than level 32, you don't need to switch and can save your mana.

Perfect, that's what I was hoping for, thanks!

What I need to try next is keeping a good charmed mob for a longer length of time. I've usually just been charming mob 1 to kill mob 2, leaving mob 1 with ~5% health. I then break charm and finish off mob 1. I'm wondering if it is worth it to keep a pet healed, buffed and equipped with weapons to tackle multiple other mobs.

I am on a break lately but I have been 34-40 in Kaesora with my Necro and I've never healed a pet ever, it's far too much work and most of the time you can get away with both mobs being even health, if your pet is dying way faster than the other mob, just throw a dot on it. Sometimes you get bad luck but generally, your method is perfect, assuming you are breaking charm with invis on yourself to make sure you get full XP from mob 2. I dont have a circlet of shadows so sometimes when trying to invis break myself when the mob is 5% I mis-time it and it dies anyways and I only get 1/2 the XP, with practice and staying in the same area with same mobs you will memorize the general damage patterns for each mob type though.

Good luck.