So many people here telling you to break up, calling it an irreconcilable difference… Reddit moment, I guess?

A couple of things pop into mind:

I think she would be extremely sad if she stayed with me and if we didn’t have kids

You think, but would she be? That’s something the two of you have to talk about. Essentially - you both have to know what the two of you want. Don’t be afraid of honest conversations, it’s what relationships often need the most, even if they can get pretty uncomfortable. Maybe she’s not super keen on children, either?

I hate work culture and I don’t want to work until I’m 80 because I have kids, it’s the biggest issue for me.

Just because you have kids doesn’t mean you have to work until you’re 80 - but what I understand from that is you don’t want kids unless things are financially stable, which is very reasonable. The question for me is: would you be OK with having kids if it wouldn’t make a difference in terms of your financial situation (or would you maybe even want them then)? Maybe there are solutions to financial issues, and it might be worth addressing them even without the question of kids. Again, something you need to talk with your gf about.

I want to live as frugally as possible.

Question is, do you want to live as frugally as possible out of principle or because of a tight budget? If it’s the first, then that could get difficult when it comes to having kids, since it’s something you should avoid imposing on your kids (I’m not saying smother your kids with expenditures, that’s wrong as well - you need to find a proper middle ground). If it’s the second, then as mentioned above, it’s something you should look at more closely, with or without the prospect of kids.

in addition to that I have hobbies and I want my time spent on my hobbies and interests

That’s a more difficult one. You will have less time for your hobbies, but there definitely should be some time left for your hobbies and interests (I’m assuming we’re talking about a couple kids at most). And who knows, maybe they’re interests you can share with your kids, depending on if they’re interested? Then you can even use that to bond with your children later on.

I don’t know what I’m looking for by posting this

The impression I get is that you know neither what you want nor what your gf wants. And that’s something you should change. Talk with each other openly. Explain your concerns. If you’re not sure about what you want, let her know about that. Ask her what she would like, how strong her preferences are, etc… make it clear that not everything has to be solved in one conversation, take as many as you need. A number of short, honest conversations are better than one long winded one. Give things time to cool down. Sleep on things. And maybe continue to make it a recurring thing, even if this topic is resolved. Communication is one of the most important parts in a(ny) relationship.