Something I mooted on here back in 3E days was using the PC’s current HD as the damage die. So a fighter would do a base of 1d10 and a wizard would do 1d6
I like it when they can; I also like it when they don't have to.
I'm mostly familiar with 3E and before so I'll go from there. It's great that you can create a NPC or creature from the ground up with all the correct HP, abilities, and whatnot. That's fantastic when you hand the character sheet...
Isn't that a bit boring? For a high-power hydra that should be happening anyway. But if you've got one or more heads flying around they can bite, distract, grant flanking bonuses, etc. They change the battle. Anyway, let us know how it goes.
So charge him an invocation and let him (q.v. Agonising Blast). Or make it an alternate class feature (add PB instead of Str / Dex mod - but not both). You know, the old warrior may no longer have the strength he once did but he sure knows how to land a blow!
IMHO it doesn't matter if they're training wheels or not; what matters is if the players are having fun. It seems that this player is not.
It's absolutely fine to define the tiers as Training 1-4, regional heroes 5-10, and national heroes 11+. Yes, it's a higher power game, but if it's fun it...
How would you model the war flail in D&D?
I'm thinking d10 or d12 damage (because, you know, the damage is swingy), Reach, Bludgeoning or Piercing damage (depending upon whether the flail is spiked or not) and eligible for the Polearm Mastery feat.
That's what's called a plot hook. You can have the low-level PCs encounter more powerful adventuring parties early on. Maybe the more powerful parties hire the PCs to do something. Maybe the PCs seek out the powerful adventurers for a Raise Dead / Resurrection / Legend Lore / other powerful...