Bonding question

I have a 1.5 year old mini rex spayed female (Luna) and we finally got an additional rabbit (Mooncake, neutered 7 month old male) and because we haven't bonded them yet, he's actually in an NIC enclosure and Luna is free roam because she has always been. A note to add because I'm not sure if it's relevant but Mooncake is really small. His parents and siblings are Luna's size but he's literally 1/3 the size of them and hardly weighs anything. (doesn't stop him from eating like a big rabbit though haha)

I actually have him in close proximity to Luna so they can get used to each other's existence and honestly they seem to love having each other there. Luna will flop next to his cage and chill by him all day sometimes and they look at each other a lot and it all seems calm and fine but once I take them to a more neutral place in my apartment and try a bonding session, all hell breaks loose. It's mostly Mooncake starting it (idk if he has short man syndrome or what) but he will attack/bite Luna and she will obviously start fighting back. We've only tried this twice and the last time, he managed to take a big chunk of fur out of her (the literal 1.5 seconds this all happened in) so we haven't done it again but I'm planning on trying a stress bonding, maybe put them in the care in separate NIC 1x1 blocks next to each other but I was wondering if this wouldn't achieve much since they're used to each other through an NIC wall already. They're not super close all the time but pretty close.

Anyone have any experience like this? They flop a lot around each other and seem to trust each other as long as he stays in the NIC cage I built but I'd like him to be free roam too. I've been letting him free roam another bedroom here and there but if there is anyway to successfully bond them to free roam together, that would be nice haha

Given that they've shown such aggression already, I would not recommend trying to "stress bond" them. That's not likely to help much and if it did, it would likely be just a temporary truce that would lull you into a false sense of thinking they are progressing. The fallout could be even worse. (more on stress bonding at link at bottom)

When rabbits are separated from each other with a pen, it isn't uncommon for them to lay near each other. This can mean 2 totally different things. One is that they like each other and want to be near each other. BUT, it could just as easily mean just the opposite. It could also mean, "hey, this is my territory and the line is drawn here. Don't think you can cross this line."

Having such extreme aggression at both attempts so far does not bode well (I'm sorry to say). You mentioned that these 2 bonding sessions were in "more neutral" space. Was it truly neutral? Where neither rabbit had been before? If it isn't completely neutral, that can cause issues too. It can be very important (especially with some bondings) to have a space that is totally and completely new.

If you try again and have the same disappointing results, you can either stop for awhile and continue with the side-by-side pen thing. OR, you may need to separate them completely where they cannot see or smell each other for a couple weeks. The idea will be to hope they forget that they don't like each other and then try again.

Not all bonding attempts are successful. Sometimes 2 rabbits will refuse to get along by no fault of yours. If you got your boy from a rabbit rescue, they typically allow an exchange if a bonding isn't working. If this is the case, I'd at least be in touch with them to let them know that you are having difficulties. That way they will be prepared if the bond ends up not working out.

Here's an article on stress bonding you may find interesting:
https://www.cottontails-rescue.org.uk/stress-bonding-what-does-it-mean/

it was done in a room neither of them have even entered (my son's room) but now that's kind have turned into Mooncake's place because I don't want to keep him in a cage all day every day