[Battle] Smaller power gap between levels

In my project I am working on I'm trying to make it so that Pokemon that are several levels higher aren't vastly stronger than Pokemon only maybe 6 or 8 levels behind.
Normally fighting a pokemon a few levels higher can be a death sentence one on one but I want to change that.

The way I was going to achieve this was by rebalancing pokemon base stats to be much lower overall.

I changed the base stats with YAPE for both mudkip and ekans to:

HP: 10
ATK: 10
DEF: 10

I made a test to see if it would work. I pit a level 14 mudkip up against and level 18 rattata and it absolutely destroyed mudkip.

Also both Pokemon were limited to using tackle and neither could get a stab bonus.

I thought by making both of their stat growths so low that it would make stat increases lower and thus make it so higher level Pokemon are not much stronger, but this doesn't seem to be the case.
I even tried increasing HP only for both but that leads to the level 18 ekans having much more HP than mudkip.

Is there anyway to achieve my desired affect? I toyed with the values more and even tried changing the base power of tackle a few times but it seems no combination of stat changes can make lower level
Pokemon stand more of a chance against higher level ones.

Any ideas what I can do?

I would imagine one way to do this is to have pokemon start at higher levels

Level gaps are less detrimental the higher the level, as it's a flat rate gain

A level 90 v a level 100 is easier than a level 5 v a level 10, despite the larger gap

Part of this is also pokemon know most of their top moves by 60, so they arent ramping up in more than one area