First of all, FWB is very hard and most of the time someone will become more invested than the other person. That's just life and human nature. IMO, it sounds like you're making an attempt to be upfront with your intentions and then moving on when they say they're too into you, which is the right thing to do.
Honestly, yes there is a middle ground between FWB/dating. There's also nothing wrong with casually dating a FWB and continuing to treat people well even if they're "just a FWB". Kind of disheartened by the amount of comments that (to me) read like they're saying you have to essentially treat the other person like shit to ensure they know it's a FWB.
To me, what defines FWB / casual dating situations is more the frequency that you see the other person. If you start spending 3-4x a week with the same person, that starts to feel more like a relationship vs. 1-2x a week. If casual dating is what you're looking for I'd consider how frequently you're seeing this person each week and for how long. If you do want to to be monogamous and casually date, what others are saying about limiting contact will be helpful in limiting feelings.
As some others have suggested, another way to achieve this is by being a bit more "ethically non-monogamous" if you can, i.e. casually dating more than one person at a time. This can help reduce frequency that you see someone, and may help your congruency of "I'm not looking for anything serious". Continue to be upfront about that if you do, make it clear as part of your boundaries space that you see other people (obviously also be safe and get tested regularly, don't want STI situations).
I think that the only other advice I'd mention in terms of how you treat your dates/FWBs, I think you should stop paying for the bill all the time. You don't have to give this up entirely, but by making them pay more for themselves, you're removing the provisioning aspect from yourself that tends to be associated with a boyfriend / more serious relationships. Not guaranteed to solve everything, but theoretically should help reduce the commitment vibes you're giving off.