S.B.
I would start the bedtime routine with the baby even before you put him in his own room. Get him used to it now, it will be an easier transistion.
When mine were little, I put the baby down first, around 6:30 or 7. My older one stayed downstairs, usually with Dad while I put the baby down. If Dad wasn't home, I let her go into her room to play or "read" while I put the baby to sleep. We did out bedtime routine and I put the baby to bed. Then, the older one and I had some time together before she went down at 8. It was nice for her to have that alone time without the baby. Then, we went to her room and did her bedtime routine with me.
I used to give them seperate baths, but once the baby could sit up really well, I started putting them in at the same time.
Now they are 5 and 3 and they go to bed at the same time. They get a bath and then they each go to their rooms and wait for stories and stuff. I alternate which one gets a story and tucked in first. They both get alone time while I read to them and tuck them in. They both go to bed around 8-ish.
Good luck! It will take time, but eventually they will get on a good schedule for you!