You should make Enemies stats upgrages multiplicatives because right now the biggest problem making infinite run possible is that enemies stats are aditives and so someone can always take "movement speed increase" and the ennemies will stay as cannon-feeders doing no damage all the run .
Making stats upgrades for enemies multiplicatives would prevent players from always taking the same upgrade and greatly augment the difficulty increase in late-game.
Yeah, you're totally right. Currently they are a percentage increase of the base value, (base * (upgrade1 + upgrade2 + upgrade3 + upgrade_n)) but I think instead they should be cumulative (base * upgrade1 * upgrade2 * upgrade 3 * upgrade_n). This may require a lot of other balance tweaks to make it feel right again though.
Really appreciate the thoughtful feedback! I'll post a devlog once I've made this adjustment.