Got it! Sorry, when i read your description last night i wasnt sure that part was done. So theres good news and bad news.
Good news: you built it properly.
Bad news: i feel thats a design flaw on ikeas part. They need to have something that compensates for that aspect.
I have a unit that has this exact problem. Honestly though, its the main solution you walk into the department and see first and you cant tell that there is an issue. It looks flush all the way down. Its just the perfectionist in me that wants it to be perfect.
Your solutions:
remove the top row of cabinets and rails, get a 1cm thick piece of wood, mount that to your wall, then attach the suspension rails to that. To make things simpler, you can just do it for the 71" span that is the center of the unit. You would have half of your 47" frames on top be regularly mounted with the black plastic spacers, and sitting on top of your tall cabinet. Then attaching the suspension rails to the 1cm strip of wood. This may be problematic because it might not fit perfectly flush in some aspect still. Im not really sure, im not that handy, i can just fix besta's pretty well. The benefit to this route is that you can now easily run lighting since theres a slight cap to run cable. This may be the lesser of 2 evils.
cut the baseboard like someone suggested. Ive removed baseboards before to replace. Its not as scary as it sounds. Its basically a decorative stick. Use this video to help with the process. Make sure to mark off where your cabinets start and end before removing it. Then cut them down to size with a hand saw and replace.
depending on how bad the difference is, you could just leave them imperfect. Its not that noticeable to someone who didnt put it up. But if it really bothers you, go for the first two options.
I know its frustrating feeling like you finished a project only to go back and do more work is probably one of the most frustrating things to deal with. A lot of people go for the fully enclosed tv unit look and i can only imagine what some of them do in your shoes that dont use reddit. I hope this helps you get to where you want to be. Good luck on the project!