>you need to be able to kill them in order to survive and continue the game
Well duh, I don't mean "spawn behemoths randomly", I mean add variance in the behaviors so it's not the same shit every single time. For example add an enemy that charges at you at high speed and explodes on contact. You'll have to use either a machine gun or a shotgun on them to pick them off quickly, but those weapons are weak against biters with such ridiculous defense/HP. It's not that you can't go there with the wrong weapons, it just becomes much more inconvenient.
Add a worm variation that spews toxic gas that floats in the nearby environment for a while. Add another that shoots 8 spikes in a circular pattern and you have to dodge them.
Add second spitter type that shoots non-homing projectiles from a long distance that can be dodged. Add third spitter that tosses high damage grenades which explode after a few seconds.
Add enemy that slowly floats around randomly and damages anything nearby with electricity, but never chases you, allowing you to avoid them.
Add underground crawling mole enemies that are hard to see visually and have a strong flamethrower resistance, but easily killed by explosives and land mines.
Add a tanky enemy that intercepts all projectiles that passes through and takes the damage onto itself, but is mortally weak against flamethrowers.
Enemies that are paralyzed by disruptor drones or gas grenades. Enemies that prioritize attacking your drones. Enemies that flee from fire. Enemies that are on water and never come to the shore. Enemies that become much stronger or become dormant depending on time of day. Enemies that randomly stop moving and attacking and withdraw into their shell for a short period, giving them incredible defense for the duration.
None of this hinders your ability to progress in the game, all it does it make the combat aspect of the game more varied and interesting. Of course you won't survive without more weapons later on, but that already applies to Factorio right now. Good luck surviving against a horde of Behemoth biters without flamethrower or rockets or combat drones or fucking nuclear grade bullets.