Sousou no Frieren

[MFauli]

It is about supply and demand.

We already established they need a healer, but magic cannot heal since only Holy spells can, so acquiring that isn't an option.
We can infer there is no invulnerability or immortality spell because no other Class 1 mages are invulnerable or immortal (They are aging and one of them got chopped in half. Others know they can lose in battle. Maybe they are all quirky like Fern and got toothbrushing spells lol).
Frieren and Fern already have the destructive power they need. They've been destroying stuff just fine the whole show.
Frieren and Fern are women, and somehow in anime, that means they are always trying to be clean and smell good.
Frieren and Fern both hate chores. Fern only does them because otherwise Frieren would rot in one place for years.

There is no supply for healing or immortality spells. There is no demand for more destructive spells. There is high demand for auto-cleaning clothes, and somehow there was supply.

There is also the aspect of utility.

Having a very powerful spell that they may or may not use one day versus having a spell they use everyday and makes them happy and comfortable their whole trip?

They chose the latter with reason. You may not agree with them, but it wasn't an objectively incorrect one. People choose comfort and ease over longtime investment all the time IRL and fiction, after all.

But that's all an after-the-fact argument. All of the things you listed happened, therefore you create an answer that encompasses all of them. That's not wrong, but incredibly convenient.

Let me ask this: Why are there no mages trying to create healing spells? New spells can be created, as Frieren proved when she told us about how mankind adopted new spells to fight demons.

[Y]

Let me ask this: Why are there no mages trying to create healing spells?

"There are, they just don't succeed" seems like the most likely answer.