Eggs are what you get for killing Guardians and the Reaper, or purchased from the merchant. They provide a small permanent bonus to 1 of the active character's stats. Egg farmer is just a massive gold farmer that you can use to repeatedly purchase many eggs on both themselves and then other characters for the stat bumps. But it really is a different kind of build.

While, the Grim Reaper isn't really THE goal of most runs, just getting to him is enough to let you get several achievements, but getting that first kill is sweet, and you don't even need to cheese it anymore, it takes a few weapons plus other decent DPS.

The achievements really are the general on ramp to most of the game, rather than just getting to 30 mins, look at them, figure out a few to try for on the next run and then trying to solve that puzzle. Getting to 30mins with weapons you've used on characters you've used in a map you've already beaten without some other goal, can definitely be boring to do repeatedly. But surviving just 18min on Bat Country to unlock Chaos Malachite can be a real challenge if you aren't well leveled/lucky/skilled, preferably more than 1 of those.