Considering there's no grinding (aside choosing your path), and you obviously walk around with your best party, if you get annihilated, or even getting seriously hurt, thus need many battles of healing, you'll need quick access to high level minions to advance.

So, assuming you had bad luck at the last battle, what can you do? I assume you accumlate a megaton of low level parts. Through eg. Alchemy, can you craft them to Emergency High Level Parts for example?

Or if you loose a couple of times badly, you get stuck for, dunno, choosing the wrong party, rising the wrong stats and so on, loosing countless hours of gameplay?

A friend of mine let me play the game a couple of hours, and I've noticed the True Ending Expansion is coming out, at a discount at the moment, so maybe someone can give an answer, so I can decide if this attractive product worth it?

I would say that no, you don't get 'megatons' of low level parts. The in-game screens even say "resources are always limited," which is true, they are.

And if you play at the higher difficulties, then you rarely get parts, and parts become extremely scarce.

If you lose too many minions and are then unable to build more minions because you have no more parts, then yes, you lose the game and your many hours of gameplay and have to start over.

I think that perma-death mechanic is a very bad design decision that they should remove from the game, or at least make it optional and give the player the choice to turn perma-death either on or off. They could even make the choice come in the form of allowing you to grind in a side-dungeon when you need to get more parts & brains in order to avoid a game over screen that would be coming soon if you don't choose to grind (but as you said, currently the game does not have any such grinding option, even though it should).

One thing your OP is incorrect about: it's not 'parts' that determine your minions' levels, but rather the brain item specifically. Other parts (that is to say, non-brains) give you more stat points on your minions if they are not the lowest quality tier of parts, but they don't increase the minions' level, with the level being the main & most important factor that determines how strong they are and how many abilities they have.

But with brains, you run into the exact same problems which I've already described: they are extremely rare, so if you lose battles consistently, then you will run out of the ability to make new minions at the same level as the enemies you are facing, and thus experience perma-death. For example, if you are facing level 15 enemies, and you have no brains, then all the minions you create with your parts will be at level 1, and therefore they are going to get quickly annihilated when you take them into battle vs. those level 15 enemies.

You can afford to lose the occasional battle here and there, but eventually it would end your campaign if you keep losing too many times in a row. And on higher difficulties, you have much less leeway for the amount of times you can lose battles and still recoup your army afterwards.