It sounds like it's not for you - marriage. I think there are risks, too, that your partner's circumstances will make marriage seem like the only safe, hopeful, or rational outcome to them. They have trapped themselves in something which is insecure.
If you're paying all the bills and they aren't contributing to a mortgage or anything, they will have got used, after eight years, to an easier discomfort-free style of living - the only fly in the ointment being it might end, and then they would be outside with their smaller income, looking for somewhere to rent or buy; bemoaning irritations like broken shelves and car MOTs looming, which they didn't need to think about before and which will now on be giving them that dull, boring downer.
If you split, it is likely to be bad-tempered. You will find it hard to explain why you don't just want to marry her because it's the natural next step. She will have got used to a lifestyle and probably has spent her own money on makeup and holidays with you and hasn't assiduously saved.
You'd be best off, being brave on a way, and saying what posters have said here: that marriage is a legally and financially binding contract, and when it's weighted against you gaining, and more likely you'll lose a lot, be there divorce, it isn't worth it for you.
It is likely you will split, as there isn't enough undying love to make you want to give up everything material to chance, for their sake. And I think you think, underneath it all, that they might divorce you later, that's why you don't feel good about taking a risk. You're explanation makes clear you don't feel that their love has been the wind beneath your wings making your flight to success all possible, and impossible without them. Your partner just seems to be there. If you did go on to marry, they might not like you much for having made them wait.
If you leave your relationship many more years you'll be into the territory of leaving it too late for them to have children effortless with someone else.
It might be nice if a mature discussion goes well to gift them some money to help them to get a place of their own or not to be in trouble. Not the case if you were an average earner, but you do seem to be very well off. It would just be a nice thing to do out of empathy for their life and respect for what they've given to your life. You can't go on for years, I don't think, with one person really wanting marriage and the other not.