Nice thoughts.
I would change it a bit.
I would have diminishing XP starting at lv22.
(Maybe have it ramp up to full diminishing status at 20% of the old dimishment scheme per level, so 1/5 diminishing at 22, 2/5 at 23, 3/5 at 24, 4/5 at 25, full at 26 and beyond. This is not the amount diminished, but a proportion of the regular diminishment, to clarify.)
Also have xp requirement ramp up to regular amount plus 10%, cumulative. So 21-22 is +10%, 22-23 is +20%, 23-24 is +30%, 24-25 +40%, 25-26 +50%, 26-27 +60%, 27-28 +70%, 28-29 +80%, 29-30 +90%. Each of the +% is of the previous amount required to level, not the regular flat amount that it would have been.)
The bonus HP at start is kind of like the new D&D/DDO thing where you get the 20 point "hero bonus" at the start.
Not sure if I like it, but the logic is good for "Game Terms", not so much for simulation.
Perhaps perks gain at lower speed after 21.
Say for regular they would be at 25 and 30, for Skilled at 26.
Skilled would be able to pick Tag! at lv21, regular at lv 22 (squat it 1 level from 21 perk).
No Terminator, Sniper, Slayer unless at 1/2 or 1/3 strength.
HP gain beyond 21 could be at half amount (getting old...) including Lifegiver bonuses.
Also, skill point gain at half amount starting at lv22 or 24.
Make Educated available at lv4. (Squat 1 lvl if not Skilled). Requirements waived after lvl 9, so anyone can take it later.
I think this would be reasonable.
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--PS Bring back Scrounger and Flower Child.