I didn't have very much time for useful comment previously, so I'm taking a second crack at this:
Any 4 skills doesn't seem reasonable. I'd reduce that to at least 3, and narrow down the options.
I'd have Stamina Surge work in the same manner as Bardic Inspiration. Give it as d6s that refresh on a long rest at 1st level, and then allow them to refresh on a short rest starting at a later level. Also, increasing the size of such a weak feature isn't enough when leveling up by itself.
Tempered Skin starts out really weak and then ends up very strong. I would limit it to resisting nonmagical damage of the two damage types you're allowing at level three. You can then have it apply to magical damage starting at a higher level, and add some other options at an even higher level. I think allowing slashing, piercing or bludgeoning as part of the class step too much on the barbarian's toes.
You only actually need all the
core class features by 3rd level anyway, so you can probably have daunting presence at 3rd instead of 1st.
Compilation of my suggestions:
1st: Stamina Surge (d6) Social/Exploration Feature
2nd: Tempered Skin Social/Exploration Feature or Fighting Style
3rd: Daunting Presence, Practice
4th: ASI
5th: Extra Attack, Stamina Surge (d8), Font of Inspiration Knockoff
6th: Tempered Skin Improvement
7th: Evasion, Practice Feature
This should move you closer to the usual power curve of a 5e class, and you can add Magical Tempered Skin and knockoffs of evasion at higher levels. I'm basing the above entirely on the existing class features, although it might not be a bad idea to make Tempered Skin part of an anti-caster subclass, and move the Defender's features to the base class.
Your Subclasses don't seem to have any social features either.
For the Defender, you should probably clarify how shieldbearer works, and Like This is a bit weak for an 11th level feature.
Cutting Board is flavorful, but gimmicky to use, for the same reason the ranger's favored enemy is a bit weak; the DM can deny the feature entirely with the monsters they use. Otherwise ,the Meat and Ironclad's class features are kind of bland. You're also missing the Ironclad's 11th level feature.
My suggestion from the ironclad is minor damage reduction, in the manner of the Heavy Armor Master feat. That feat is noticeably stronger in practice than on paper (at least if your DM understands the action economy), so you can go ahead and make a subclass that stack with it. A lone man standing against the horde is a really cool scenario, and much easier to achieve with damage reduction.
Two subclasses of "guy who takes hits from melee weapons" is enough, and you've got 2 of that in your 2.5 subclasses. Alternate Ideas:
- An anti-spellcaster who can cast Counterspell once at 3rd level and multiple times at a higher level. Can be combined with Tempered Skin if you make the Defender's features part of the base class.
- Someone who can create areas of wind that interfere with ranged weapon attacks.
- Someone who can summon minions with their own limited tanking ability.
- A healer subclass, restoring damage it fails to redirect at itself.
- A subclass that reduces enemy movement to 0 if they fail a saving throw (or inflicts fear, at higher levels).
- A subclass that grants variety by giving features other than tanking ability.