[RPG] Are there any existing magical/artificial replacement arms written up

First and foremost it is important to me to mention that those of us that play this game for the story and as you say "correctly" (although it is a bit of a misnomer given that there is not an "incorrect" way of playing) the story of the character is and of itself its own reward.

I played an evil Halfling wizard once, for a single session. He was out for his own gain and in the pursuit of that somehow thwarted the assassination attempt on the rest of the party, to whom he had not yet been introduced. He ended up dying pursuing his goals. My reward for playing my character? There is a statue outside the library in honor of my "heroic" sacrifice for the lives of the others. I am touted as a hero, sort of like the Ballad of Jane from Firefly but this was way before I ever saw it.

Rewards I have used are:

Bonus XP: I usually don't do this anymore but it is an option and I know other DMs do this a lot. I will still give bonus XP out occasionally, but it is a bit tricky at times if the others slack too much and lag behind more than intended... so be careful with how much at a given level if you choose to use this option.

In game advantages: I have started to prefer this approach, in this way you gain tangible advantages by how you treat people and with whom you interact. This gives the world a feel of interconnectivity as well as realism, that what they do matters. If your other players are raping and pillaging and treating the NPCs like dirt they should expect to be treated with reciprocity and if they see that your ideal player is getting free room and board for saving a kid from a runaway wagon or offering to work in the kitchens, they might be inclined to switch tactics. On the converse if the evil characters that are the bullies are playing well you can have them be approached by contacts for them as a reward for that sort of thing.

Inspiration: Which you indicated in discussion that you already use judiciously but was removed as a discussion in the comments of the original post.

Some players drive the story others are along for the ride, by making his character the chief piece in a story is a good way to reward him as well as set the example.

What I would recommend is that whatever reward you decide on make sure it is available for everyone and everyone is told why they are given this or that reward. Even if you hand out XP for bonus and awesome play be sure to itemize that with what this piece was for and that piece. This allows them to see for them selves what their efforts glean.

The answer to both of your questions (this and the parallel one) is the same:

Stop giving all of the NPC attackers the same initiative.

That step right there eliminates the focus fire problem. That choice is the first element of a two part solution. You are the DM and you have this choice.

The Cleric is getting to 0 HP quite frequently, though, due to burst damage caused by all the NPCs attacking at the same time (as stated in the other question) or simply being unlucky.

If you break up the attackers in to smaller sub groups, this burstiness is mitigated.

Now for the experience based part of the answer.

The DM in our first campaign in 5e taught me a valuable lesson; he broke the NPC enemies into groups once the number of enemies were larger than 3 or 4. He also had leaders, or different kinds of monsters, roll a separate initiative. All but one of my other DM's have done the same. You can watch the difference it makes at the table. A side effect of this is that the battle gets to look a little more like a tennis match: a couple of their guys attack, a couple of our guys attack, etc.

Example: seven goblins and two ogres fighting a party.

• Two groups of goblin initiative (a clump of 3 and a clump of 4) and either separate init for each ogre, or both the same.

• For a level 2 party, I'd spread it out because Low Level D&D 5e is swingy.

  My advice on "the dice are fickle" applies to this also. This approach spreads initiative out so that party actions are interspersed with enemy actions. It's still swingy, but it is less bursty for one side also.

Second element of a solution: a primer on tactics for your players

Sit down with your players and discuss tactics. While each group has its own opinion on how close to "combat as war" they want to get, your choice of playing "smart monsters" (IMO a good one) means that the players have to learn

1. how to apply their tactics As A Team, not as individuals

2. how to play smarter not harder.

   Sometimes, it takes some DM coaching to get them working in that direction.

   A tactical point to discuss with your players for this particular problem at 2d level: what are the spell casters doing to slow down the enemy? What crowd control or "dividing the enemy" measures are they taking?

You are the DM, so coach them. Almost every DM I've had over 4+ decades with this game has done a little, or even a lot, of coaching. It comes with the role.

As a final observation on your problem: if, even when faced with the facts, your player still thinks you are out to get him, there is only so much you can do as the DM. Some people are simply like that. My suggestions above may help mitigate that so that this player's perception changes.