Am confused - what benefit is a better brain to us right now, or in the very near future? What are we unable to do right now, that a mouse-like brain would let us do? What do you believe this mouse brain will give us?
Brains are for later, I really, really believe. For when we're trying to do stuff that requires our critter, -not us-, to understand what it sees. Like, if we have to understand the behavior and constructions of sentients. Maybe we should grow a better brain, but honestly - don't we want a better brain than a mouse brain? If we take a mouse brain -now-, then when we get a better brain later and want that one we've effectively wasted our evo and food spent getting the mouse brain.
We seem to be surrounded by food right now, ungaurded food. There's these Fermroot plants, there's other plants, there's plants and debris in the water, there's various critters in the water, and there may be remains from a welif corpse or a wounded welif itself still around when we're ready to consume more, as well as those blessedly distracted mice and all the other organics around this ecosystem. We may be surrounded by 100s of points of evo and food.
The welif's agonies should distract and attract most local scavengers/predators to it. We're by some Fermroots, which are disgusting to most critters around here. Let's move beside/beneath one of those Fermroots, perhaps curl around its base as 'shelter' for our current evolution opportunity. Why not move further away? Because scavengers and predators are currently moving to the general area. Everything is alert and watching the general area - and if we haven't been overly noticed yet, continuing to move increases the likelihood we will be. We're on the other side of the water now, in unknown territory, and we'd have to move deeper into the unknown - which may not be a better place to evolve for a long distance.
I think our most important short term priorities are improving our skin digestion efficiency further and our ability to store food. These things will help us in absolutely every future feeding opportunity, including the possibly inevitable pauses between meals that may occur. The more efficiently we gain food now, the more food we get from this bonanza and into forever.
I believe our most important near term priorities include toughening the skin we don't intend to eat with, improving our sensory capacity (color vision, smell, hearing... I think we're getting some touch information so maybe not that), improving our body's capacity to contain more oxygen (so we can be underwater for longer time periods/far later possibly be in other 'there's no air!' situations with less distress and pressure), and improving our mobility both in the sense of -super fast- motion like our welif's ambush snatches and in mouse-like movement speed and climbing ability - which may or may not require body redesign to achieve this... I rather hope it doesn't. Leg redesign, maybe.
After we've moved comfortably close to (one of?) the nearby Fermroots and settled, observe the surroundings for a short time to ensure we appear 'safe for now', then improve skin digestion ability, primarily for greater speed and greater efficiency both.