We are talking levels 17 to 20, the "Legendary" tier?Is having every spell in your spellbook heavily modified so that concentration breaking doesn't matter anymore, friendly fire doesn't matter anymore, energy resistances practically don't matter anymore..... is that broken or just "the expectation of high level play".
I dont see a problem.
With regard to game engine design, the purpose of "Concentration" is to prevent a mage from layering too many powerful spell effects at the same time. That important restriction continues to remain true even in the highest tier.
The possibility of interrupting a Concentration spell is mainly for narrative flavor. By level 17, I hope every fullcaster has already made an effort to shore up Constitution saves along with Concentration protecting features, to make negligible the possibility of any interruption.
Preventing friendly fire is powerful. But it doesnt actually add extra damage, and in that sense is balanced enough. Meanwhile to leave allies unharmed by a Fireball is great flavor, because it dramatizes the mastery of magically harnessing raw explosions. This is a "Legendary" Wizard indeed.
Bypassing energy resistances is fine at this highest tiers, when most opponents exhibit some kind of damage resistance. Meanwhile, the Wizard would still need to prepare several versions of the same spell, in order to benefit: Fire-ball, Acid-ball, Cold-ball, Lightning-ball, etcetera. To redundantly use up the number of possible spell preparations, is a wash.
Again, I dont see a problem.
Every spell needs to calibrate that so that it is roughly balanced with every other spell that is in the same spell slot.Shrug, its a massive power boost. When people talk about the Simulacrum cheese, they still consider it broken even for that high a level.
A spell that is underpowered either needs a boost or else a demotion to a lower slot where it is more competitive alongside the spells there.
A spell that is too powerful either needs a nerf or a promotion to a higher level.
Each spell stands on its own. Designers needs to doublecheck every spell, on a case-by-case basis, to ensure it meets the expectations for the level it becomes available.
Regarding Simulacrum specifically, its description might need more cautious wording. Personally I havent seen players abuse it, but I can see how one might.