Help with sleep after transition out of crib

We switched my almost 3 year old to a twin bed about a month ago and my 8 month old has been in the crib in the same room. I still nurse my 3 year old just to sleep. I want to stop so badly but waiting for my husband to be off for 3 weeks in April to help me with that. Baby wakes up multiple times a night still. Towards the end of the night I usually bring him into bed with me. Wish I could sleep train but I can’t bc he’ll wake up my toddler. Toddler was going to sleep himself and sleeping through the night in the crib. Now he will not go to sleep without me in the room. He also wakes up sometimes during the night and comes to get me and expects to be nursed back to sleep. Then he wakes up at 5 am and won’t go back to sleep. He comes in our room and keeps all of us awake until we get up. This morning he was screaming for boobie until I gave in. Then at like 4/5pm he gets super tired since he woke up so early and has been having raging meltdowns that sometimes last an hour.

Other notes:

I can’t lock him in his room, not only bc I don’t entirely agree with that approach but bc he will also lose his shit and that will be traumatizing for everyone.

He doesn’t nap anymore. Haven’t been using any screens as I know both of these things can affect sleep.

We have the alarm clock that turns red when it’s time to sleep and green when it’s time to wake. It also has eyes that open/close. He understands the concept bc I have pretended to take a nap during the day and he tells me “no mama eyes open” and points to the clock. He has also woken up at 4:30 and then after being up for 2.5 hours, comes running to me excitedly saying “mama eyes open! Light green” 🫠

Any advice on how to navigate this situation and get everyone sleeping better would be appreciated. We are so tired.