Hi,
I just wanted to share my personal experience in case it's helpful at all.
I got my two IRNs when they were almost 4 months old (and weaned). They had been handfed by their breeder, but beside that, had not been handled much at all and were INCREDIBLY terrified of me. They would flee to the farthest corner of their cage, and if out of the cage, would flee even farther!! I initially sat by their cage a lot and talked to them and would lure them over with food, and they would eventually come over to take food from me. One of my boys (Marcel) was the first baby and the only baby in his clutch, so he was slightly less terrified than my other boy (Ollie), who was one of a clutch of 4, and they were as wild as wild could be.
I think it took about a week or two to have Marcel easily step up on me and be comfortable. I worked with him in a smaller room (without mirrors or windows!) in case he flew, so he wouldn't be able to get as far. Meanwhile, Ollie would bite my hand if I got near him. He would usually just fly out of the cage on his own when I took Marcel out. (He would, of course, take food from me though, haha). This continued for a long time. I never forced them to come to me; I would only lure them with food (most often sunflower seeds). They would be rewarded with a seed for going back in their cage too. That's' how I started getting Ollie to step up on my forearm--however, any attempts with my hand would quickly result in him chomping down. HARD. Frequently, Ollie would fly to me (of his own volition) and immediately put his head down and chomp. For no reason. It was incredibly frustrating. Most things I read seemed to say that a bird would bite because it was threatened, but this was completely unprovoked! I grew up with parakeets and had a Senegal, so these were not my first birds either! But I had never realized that handfed birds could be like this!!
I read pretty much anything I could about how to deter biting. I tried training them to fly to me and tried a few other simple tricks to build a positive relationship while also distracting them from just biting. SLOWLY, it got better. They got very good at flying to me on command...the other tricks, not so much...haha. I did start to get better at reading their body language. I noticed that in general, both of my boys like to nibble. Part of it seems to be a very ingrained behavior that's part of their natural display, and I learned to differentiate between when they're going in for the (harmless) nibble or going for the CHOMP.
For the biting, I read a lot of things. I seemed to frequently hear people just saying to ignore the biting and it will eventually stop. As I am a musician who needs my hands and can't risk nerve damage, this was not an option for me. If they bit, they would immediately be shaken off and I would verbally scold them in a firm voice (my birds are flighted, so they were never at risk of falling or being hurt). Personally, I found the wingsNpaws youtube channel to be helpful too. He talks about his observations of birds playing and biting among themselves, and how IF they are bonded to you and you make a sound like you're hurt and leave/"fly away", they start to realize that this is not a desirable outcome. I started to try this. If I got a nasty bite, I would make a pained noise, get them off, and disappear out of sight for a minute or two. They would clearly get distressed that I had left and they would actually try to follow. When I reappeared, they would always immediately fly to me and indicate that they wanted to be with me. For someone who had been taking a lot of abuse from a certain little Ollie, it was a relief to know that he actually DID like me!
I also initially had them stepping up to a perch instead of me, which was moderately successful. I also was working with a clicker, but they never seemed to grow used to the sound. They would just get scared of the sound instead of associating it with a reward, so I eventually stopped using it.
Gradually, the biting waned. Luckily, I never got a severe bite from them (my Senegal did a lot more damage than these guys ever could!). Ollie did break the skin a few times here and there, but it was never anything terrible. I've had them now for 11 months and it's really rewarding to see our bond grow stronger. They definitely have both been displaying a lot this spring, so that has been ... interesting. However, they are both very comfortable with me AND, to my pleasant surprise, are also very social and friendly with people who visit my apartment (although they tend to do better if it's one person vs 4 or 5). Within the last month or so, Ollie has actually started readily stepping up onto my hand and finger. Every so often, he will firmly use his beak, but it seems to be more of a leverage thing than a bite. He also lets me scratch his neck, which Marcel (the one who has always been more tame) will not permit at all. They both have even learned to talk a bit, but Marcel has a much better vocabulary.
All I can say is that with these birds, it seems to be about patience and the long game. Birds are smart, but they also have to be motivated to be trained. Forcing them into anything is just going to result in them holding a grudge, and causing a lot of future behavior problems (for you or for any other humans they encounter). Be as calm as you can, reassure them that you don't intend them any harm, and use positive motivation as much as possible in building your relationship. It WILL get better! It just might take a while! If you can win them over, they are just a delight
***Edited to add:
As far as weaning, if they're only a month and a half, that seems rather young to be weaned.... Mine were only just weaned before I got them from the breeder and they were almost 4 months. I noticed that this spring, when following my breeder on facebook, she just sold another baby IRN weaned at maybe 3-3.5 months. I don't think it'd bad to offer them real foods, but I wouldn't be surprised if they're just not interested for a while longer. I've read that for weaning, cooked veggies or softer fruits are more appealing and manageable for baby birds. One thing my breeder did that I just haven't gotten around to is rice, defrosted corn/pea/carrot mix, and crushed hard-boiled eggs--shell and all for calcium. Good luck!