The best early upgrade is usually the one that removes the problem ending your trips most often.
Prioritize oxygen when dives end too early
More breathing margin can improve exploration and gathering at the same time.
Prioritize mobility when travel consumes the session
Faster movement creates more useful time for exploration.
Prioritize storage when every trip ends in clutter
More organized capacity can improve the value of every resource run.
Prioritize vehicle support when range becomes the bottleneck
Once the surrounding area is familiar, stronger travel options can open the next layer of progression.