Love the feeling of this mod. It adds a very nice challenge and sense of urgency compared to vanilla Factorio.

That said, to get better weapons we seem to need to kill enemy bases. But the weapons we have can't kill enemy bases fast enough? I emptied half my inventory worth of ammo using 5.56 turrets and cannons, plus about 20 rounds with the flamethrower, into just an extractor and a sunken colony and only the sunken died. The extractor still had half its HP left.

I don't think I could have carried much more ammo than that.

Maybe I'm overlooking something or is Hardcorio is just a little too hardcore in that regard?

Enemy extractors produces Elerium - quite a valuable material. So spend some resources on its destruction - quite a reasonable price. Especially before its destruction can be seen as Elerium already accumulated.
Additionally extractors regenerate health of the main building like the spawners.

I use the flamethrower for most of the work (started fighting back with modular armor). Just one click will produce a burning tile that will damage the building. So the way i kill a base now is:
circle strafe arround and only one flame on every building, when the flame goes out reignite it. It is super efficient on the flamethrower amunition.
After that i use 5.56 gun to kill the extractors (It takes a long time till you leveled you weapon skill up + some damage upgrades). When i killed them the next is the main building, again with flamethrower and setting every tile the building is on, on fire.
here you can see how i attack a base 3 times normal speed
https://youtu.be/jyfFf89mlh0?t=67

At this time code i use more ammunition than needed. If you switch to the 15 min mark you can see i only use singe flames per building.

Btw. turrets don't get the damage bonus from you weapon skill so it takes way more ammunition.

Ahh excellent advise! I've been using the flame thrower in a much less efficient way. (It's a good thing oil is infinite too, so we can use the flame thrower for a long time.) Thanks!

Tried it and it works great. Thanks!

...that said, killing a base is just too time consuming to be totally fun. Feels like work after a while. :)

Yes it does. While you kill the extractors extrem fast in the later game, die flamethrower doesn't get any stronger. So it still takes a lot of time to kill the bases. The laser/plasma based weapons are a bit to weak. And even tanks with expensive rockets don't do to much to help out.

I went ahead and built a supply line of ammunition and put up some sniper turrets. But killing the base still took a silly amount of time. Also this approach is much more expensive in terms of resources: sniper ammo consumes resources, so you're trading your finite resources for a little bit of convenience.

I'm experimenting with building a moat now. Using the explosive packs I have, as an experiment, built a water channel that blocks enemies. I'm thinking that if I take a weak enemy base an encircle it with a moat, I can come back and kill all buildings very cheaply once in a while to "harvest" alien artefacts.

Unfortunately the explosives take tons and tons of resources so I'm running out of iron ore and need to expand my base. But the thing where enemies start to spawn inside of your base when it gets larger makes that a big bother.