Chainsaw Man

[KrayZ33]

When a devil takes over a human corpse, it becomes a fiend. If it doesn't become more powerful, why do that? Why not stick with its original devil form? At the very least, it seems to remove the need to consume blood, since Power doesn't seem to need to drink blood to maintain herself. That's where I got the fiends are stronger than devils, at the very least if its the same devil becoming a fiend. Naturally, there can be variances where a fiend is weaker than an originally high level devil.

As for Pochita giving his heart to Denji, that's because he cared for Denji and wanted to see his dream, even if Pochita himself ceased to exist (in his current form) to do so. If Pochita just wanted to survive, he could've possessed Denji and become a fiend, like Denji himself suggested. Denji is NOT a fiend. His case is special, as Makima mentioned.

If they want to become a fiend, why did the Bat Devil not become a fiend?
I don't think that it's a voluntary thing, more like a last resort at best, or maybe not even that... but I don't see the drive you are talking about. I don't feel like they want to become fiends at all.
Pochita didn't make Denji a fiend nor turn into one himself, even before he and Denji became friends.
It doesn't seem to remove the need for blood either considering that Power craves it still (EP2) and that bird(?) dude they killed in EP2 was feasting on something as well.
The Zombie Devil wasn't a fiend either, even though he had more than enough bodies to transform into one, some were probably more powerful than the average human as well, both in terms of actualy physique and influence.
And I don't think Power would talk about her former self so proudly like people do when they did something special in the past, that is now gone or not exactly the same way as before.

"Before becoming a fiend, I was a devil feared by all..."
Feels like you'd rather not mention the devil part at all and say "I'm just a super strong fiend, I'm feared by everyone..."
or
"Even when I was just a devil, demonic peons would tremble in my presence...."
Although I have to admit that I don't speak japanese, so that might be a translation thing.