In the mongrel rda I've been using 3mm clapton.
I've tried the coil it came with, but switched to fused clapton coil. I added a half wrap to make it fit. That's were I got some flavor from. It is a 3mm fused clapton. I'm using the restricted insert. As for the wicking, I am using cotton bacon V2. I've ordered some firebolt cotton because I heard it was pretty good. I have a few vape stores in my area, but they don't carry a lot pre built coils or cotton. I have a couple RTA's but can not ever get the wicking right.I don't have a Mongrel rda but do have many other rda's. I regularly drip and squonk.
Looking at the Mongrel build deck it's bottom AF and there are guides on each side for positioning your coil. That makes it easy for building.
View attachment 950845
Are you running your Mongrel with the full or restricted AF insert? Which ever one you are using try the other one and see if that helps to improve your experience.
What ID coils have you tried already? Like you know, 3mm, 3.5mm ID, or what? You could try a different coil ID. I do not know how large of a coil ID the Mongrel will take. Try some different coil ID's until you find one that suits you best. Ideally you want a coil ID where the airflow is hitting the bottom 1/3 of your coil. From my experience that is the sweet spot with most RDA's. You don't want too much air flowing up the sides of your coil.
If you were using the 3mm Ni80 coil it came with and are not happy with it then try another metal, like SS316L or even Kanthal. Some folks like simple round wire. Others prefer claptons, aliens or another exotic type.
IMO, claptons and other exotics have more surface area and provide much more flavor than simple round wire does. When I switched over to more exotic coils the flavor popped. I immediately got much improved flavor.
Wicking. What are you using? Cotton or Rayon?
If using cotton you could try another brand. Not all cottons are created equally. Some are total crap while others are superior. Some folks swear they get better flavor using rayon. I switch off between cotton and rayon all the time.
In the end, it is going to come down to you experimenting with different coils and wicking. There is no ideal setup that works for everyone. That is true for RDA's and RTA's. You need to try different setups until you find what works best for you.