In terms of how to maximize first biome strats for maximizing what you have going into the second biome, here's my suggestions:
Run to the first random room, trying to touch nothing but crystals. Avoid all plots and enemies. If you don't get Horizontal Thorns, Vertical Thorns, Star, Thwomp, or Homing Seed, find the nearest spikes to kill yourself. Run to the random room again. Do this until you end up with one of those. Ideally Thwomp, because it is the seed I find useful most evenly across the entire game, but Star can be the difference between a good boss attempt and a flail, so play according to taste.
Work your way back down the first biome, trying to minimize the number of plots you use to kill the maximum number of enemies. That's why Thomp is the best at this. Use all other seedplots for hearts. Work your way up the first biome from the bottom with this method.
Ideally in the first camp, find a magnet aura. Do the challenge room, which with magnet aura and your run through the first biome should mean you have 30-50 crystals, and ideally 5+ hearts (assuming you aren't a derp like me and let your good pal thwompy hit you). Put some crystals in the bank. Buy every seed in the store. Not because you actually want them (although if they are good drops, then yay!) but to minimize the possibility of them showing up later in the game when you need something else.