There is one critical flaw in this argument: Martial/caster multiclassing sucks. Unless you rely on exploits like the sorcadin, or the wizard who dips two levels in fighter so they can nova harder with Action Surge, you will be significantly weaker than a single-classed character of the same level.
Let's look at a 10th level Wizard / 2nd level fighter versus a 12th level wizard. The 12th level wizard, in 2 rounds, can cast a 6th level spell and a 5th level when they want to nova. The 10th level wizard can cast 2 5th and 1 4th level spell. Which will have more impact? And, if your answer is not the multi-class, consider that after a short rest they can do the extra spell again. I give the edge to the multi-class in a head to head power comparison.
Feats should be judged in comparison to other feats or to ASIs. Which would you rather have at 4th level: +1 to your attack and damage rolls, or one 1st-level and one 2nd-level spell slot? It's not immediately obvious, which suggests that this is a reasonable option.
Spells of your choice, as opposed to spells preselected by race for a feat (such as drow, deep gnomes, etc... can get), allow for more optimization. In my analysis, those extra low level spells can be devastatingly powerful if cherry picked. Additionally, most feats comes in shy of the benefits of an ASI in your prime attribute. There are a few equal or above it, but most come in shy - which is why many PCs do not get a feat until 12th level.
At 12th level, your primary stat is presumably maxed out, so now you're comparing one 1st-level, one 2nd-level, and one 3rd-level slot to, let's say, the Sentinel feat*. Which is better at 12th level? Sentinel is a darn good feat, but I lean toward the spell slots, which makes me think they're a bit too strong; the power gap between 2nd-level and 3rd-level spells is far bigger than the gap between 1st- and 2nd-level. If this were cut back to just granting one 3rd-level slot, I think it would line up better.
Cherry picking the absolute best spell to aid you from 3rd level or lower is a huge benefit. Which one you'd take depends upon build, but I would find a single 3rd level spell of my choice per day to be a huge benefit, on par with the overpowered feats that most agree would have been better if slightly pulled back.
*I picked Sentinel as an example of a feat which is very strong, but not notoriously overpowered like GWM or Sharpshooter.
I put them in a similar tier, but Sentinel's benefit is controlled more by how the DM approaches a PC with the feat. In some games, it is a bonus attack nearly every round, with extra benefits to the attack, while in others it serves primarily to keep monsters spaced and away from the PC. In my games, where I strive to have monsters not metagame against the capabilities of a PC, it is more useful overall than GWM or SS (in my opinion) when used efficiently.
