I did a guide on Miko a few months ago, where I thoroughly went through his skill tree, playstyles, gear (which I admittedly should update), tips and more. If you want to see how I reason regarding certain things about Miko, you can check there.
I do want to point out a few things here though to this post specifically;
Miko’s a fantastic pocket healer and is probably the best in the game at being such a healer. Playing Miko as a pocket healer doesn’t mean exclusively healing one player, but primarily focusing on one or two at a time, your tanks/front-liners if you will.
Healing just one player? Then you need to learn to play more effectively.
Pocket healing your front-line? Then you might just be doing your job.
No, it isn’t. It’s an under appreciated helix, but ia under no circumstance a no-brainer. Miko’s one of the easiest targets to kill in the game and Heal Thyself will save you a lot. Regenerative Aura is best picked when you got another healer, against a team whose incapable of locking you down/bursting you down or you have 3/4 tanks/melees/front-lines. It will increase your healing done for the moment, but that may bring hollow if you’re repeatedly dying, leaving your team without a healer (or one of them) and feeding the enemy team exp. And while Regenerative Aura can save players from certain situations, Heal Thyself allows you to make dives yourself to save someone like Attikus, Phoebe or Kelvin or just be a part of that dive in the first place. They’re both really good and shouldn’t be overshadowed by one another.
This, again, is not a no-brainer. If you’re running Regenerative Aura, you might want Evolutionary Emergence instead since the raw heal from Biosynthesis is all you got to heal yourself and to capitalise on the additional healing granted by Regenerative Aura. Also, with Evolutionary Emergence his ultimate Fungus Among Us only has a 6 second cooldown unless you pick We Are Many at level 10.
Trail of Spores is a fantastic CC-tool to delay an entire lane, or even team, especially when combined with Breathe Deep. It’s also great to create “no-go zones” or as an initiation skill to try and lock down agility-reliant characters such as Mellka, Thorn or Deande.
Not all characters who will go for it are melees who need to get into it to kill it.
Also We Are Many makes you able to have Fungus Among Us active at all times and allows you to be more flexible and responsive as to when and where you throw it down.
This is the gear I use at the moment;
Unstable Neural Accelerant
+9.80% Sprint Speed
+17.50% Recoil
0 cost
Antiquated Stout Ekkuni Wristguard
+9.10% Attack Damage
+210 Maximum Health after surviving 180 seconds
-140 Maximum Shield
420 cost
Attack damage actually affecs Miko’s Healing Beam, which is why I use increased attack damage on him at this moment.
A primary attack speed piece of gear beats a secondary healing piece of gear (9.10%>7.00%).
The increased health after 3 minutes is a quite nice boost to his survivability as well.
Drained Survivor’s Remedy Box
+14.00% Heal Power
+7.00% Heal Power after surviving 180 seconds
-42.00 Shield Recharge Rate
630 cost
I keep the loadout cheap to allow myself to stay in lane at all times without having to leave it for shards.
All in all, I think it’s somewhat too streamlined. Miko’s my favourite character, largely because of the versatility his helix tree offers. In my guide, I made 4 different helix combination reccomendations; Single Healer, AoE-Healer, Wave-clear Support and Assassin Support. The helix choices at level 2, 3, 4, 5, 6, 9 and 10 ahould all be chosen dynamically depending on the match. The skill of the opponents, opposing team comp, your team’s team comp, mode, map etc.
Gameplay videos of me playing Miko;
Meltdown
Face-Off
Incursion
I realise I’m not thorough enough at certain points, and there are things I’d like to say which I haven’t covered here, but typing on the phone is not optimal for longer texts 
But that’s also why I refer to my guide to Miko, you can see more of my thoughts more thoroughly there on how to play Miko all in all.
Edit: Fixed the gear positives and negatives.