GIVE ME THE REAL TALK.

[Omni]

The latter switch I've always wanted to make but as I feel it lends itself to a sense of action and immediacy better.

I feel like this is a common conception, but I don't buy it. Whether you're saying "I" or "Samus," or "punch" or "punched" shouldn't affect how immersive your writing is. It comes down to how you write. If you say "I feel the cold breeze caress the nape of my neck, and my skin prickles" there's barely a difference between "Samus felt the cold breeze caress the nape of her neck, and her skin prickled" because ultimately you're describing something that the reader can relate to, whether it's happening to a first person or not. It doesn't have to actually happen to the reader, they just have to be able to insert themselves into that character.

Not to mention that the reader doesn't suddenly imagine a set of actions happening faster or more intensely because it's happening "now." I've read both and I imagine both the same way. I just don't get where this idea comes from.

I certainly don't think your first person use has made your writing worse, but I also don't believe that it's made it better. It's simply a tone change.

In which case, why do we default to past?

It makes some things easier to say, but I dunno. I feel like I need to explore this further.