I can't even hold my girls while they're eating but they trust me. They like to run to a corner and eat their snacks because that's what rats are programmed to do BUT they completely trust and love their Momma. I can shoulder ride ALL of them and pet them. The younger two LOVE to give me kisses and lick me but won't sit still sometimes and will squirm. Each rat has a different personality. Only one of my rats will run up to my face and give me nose licks. The other two just don't do that.
Maybe when they're older they might not mind being held while they eat but I don't see that as trust. If your rats are comfortable being picked up and like being pet and you're not getting bit then I'd say that's trust. Trust takes time but there's some things rats just don't like. It doesn't mean they don't love and trust you.
When I pick my rats up they will sometimes squirm to get down. I let them get down. Not everything has to be on your terms for trust.
How long have you had these rats? Did you have them from just weaned or were they adopted from another situation?
I'd start with shoulder rides to new places and allowing them to sniff other places. They will walk around your body while you walk with them. Rats love to explore new places. YOu know how their noses are always going. LOL
I wouldn't worry too much about being able to hold them upside down while they're eating. Just take lots of time with them, lots of fun things and lots of lickable treats put on your hand/fingers because they need to learn that hands mean positive things. I have had the best luck with homemade custard. Every single rat I've ever had just LOVES it. I don't put much sugar in it but it is sweetened. There are other things you can use too. Rats love whipped cream too. It's not something I give every day but when I do it builds good feelings between me and my rats.
I think if your rats like being stroked and you are hand feeding them and taking them for shoulder rides you are ahead of the game. It already sounds like you're building a good relationship with them.
OH and let the boy that loves kissing you, pick him up and walk with him while he gives you kisses and pet him.