I don't have an answers.
I'm working through the How to Talk book and it is helping. Also crawling through the explosive child. Also gauging the day and being okay with backing off/changing plans or making sure all of PHALT are met before doing some thing. Or doing it without them. We stopped grocery shopping and errands with kids in tow, we do them at night or in the weekend and tag team. My middle dd finds them way to overestimulating and it is always a circus, I am stressed and not parenting well. We do everything possible to not take kids in errands unless absolutely necessary. Unless we bring one because they asked to go along.
Clear expectations "we're doing this, then that then this will happen." And repeating each step of the way to hold boundaries. Consequences for rude behavior (you don't get to go _______ next time, going ________ is a privilege and in will not be treated so rudely ever ever again. Until you can be kind and follow our rules, we will not go _________." And then eventually at the next attempt down the line set them up for success, clear expectations and reminders before approaching the ________ situation
)
When they are rude to a sibling "whoa. Try again. I hear you feel hurt/insulted/furious about bla bla bla, but it is absolutely not ok to hit/be rude/be unkind. Try again...script." or "whoa you sound furious!" And try to connect. I have been pulling that kid out of play and pulling them close and giving them hugs and trying to help them relax or laugh to trip their hippocampus out of flight or fight so they are able to relax and problem solve."
The Whole Brained Child was super helpful with seeing certain things in relation to brain development and pathways that are glitching.
Also anger is a secondary emotion and can be anxiety related, is there some thing in A's meltdowns that could be triggered by anxiety? Too much noise? Too many choices? Needing to go faster than he feels he can? Fear of some one getting lost? Have you tried rescue remedy before stressful situations? Focus on feelings behind actions and setting them up for success, some seasons it isn't worth it to try and do certain things. What will bring peace? What is best for the kids you have?
ETA: Also, don't gauge *anything* about anything right *now*.
You're 2 months out from giving birth. You're all still majorly adjusting and every thing pretty much feels like it is falling apart the first long while...We're 10 months out and it's really in the last month where I feel like we're all starting to settle into this new family we have. I feel like it truly does feel like it takes about 9 months to hit a sweet spot, some times longer if the baby is high needs. It will get easier, you are not failing them, not even a little. You're all still adjusting to a whole new normal.