I started topping Marc up with formula for his evening feed when he was about 8 weeks old. I was just too busy with the other 2 and couldn't give him enough milk or time. Sadly for me it didn't seem to make him sleep better but it was so much quicker and easier and I least I knew he was going to bed on a fuller tummy. I would also get Scarlett to bring his bottle into me to involve her a bit more and that was something she loved doing for him. It also meant someone other than me could put him to bed.
Good luck and it sounds like you're working it all out. We still have some bad days here and it's hard to see the forest for the trees sometimes