Engagement dinner?

It's not really appropriate to throw a party in your own honor, so no, it would not be appropriate to do that, and then ask your guests to pay on top of that. You need to fully host the meal if you do this. The proposal boxes were an entirely unnecessary cost, so I'd consider returning those items, and using the money towards your dinner.

I went out to breakfast with two of my brides folks (and their SO's and my FW) and just asked them, they payed for their own breakfasts. We go out for breakfast most Sundays, it wasn't that different, only we freaked out over the wedding a bit.

You literally stated it wasn't an engagement party (which I still think you can throw for your self), you don't need to buy everyone's meal, particularly if your group does this anyway.

Could you do a bar thing? Just have everyone meet you at some time. This way it's casual, not really a party, and there's no pressure to pay for everyone.

"Since it’s not going to be an actual engagement dinner, but more so a gathering, would it be wrong to have those we invite pay for their own meal?"

That would be wrong to have those pay for your own meal. That's just appropriate hosting in general. Even if it's "common" in your circle of friends. It's rude to say here come celebrate me but you have to pay for it. Save the money from the proposal boxes and dinner and put it towards a nicer bridal party gift.

I think it would be best to pay for your friends meals. Even if it's just an informal gathering. Since you're initiating it and your intention is to get them together for something regarding you and your FS, I think you should pay for their meals.

And maybe reconsider giving them these proposal boxes, at least in front of the group. I understand the intentions behind wanting to ask this early, but maybe give it another year before you ask. That should be plenty of time for them to save what they need, without them having to worry about an event that's over 2 years in the future. Relationships can change a lot in that time. And asking in front of a big group of people and presenting someone with a box of goodies can put a lot of pressure on someone to say yes. You could always send them the box later after they accept.

I visited my friends over the course of a week and gave them a card asking them to be in my bridal party. My FH did it with facebook messaging or text.