The new workflow would work if it reduces the load on the designer.
I'm not sure I understand the full picture, but you say:
Due to the designer not having enough time, the team started to work on a lot of stories in parallel. The more stories got finished, the more work was there for the designer. [...] The designer was always lagging, developers were ...
You Have More Choices Than You Think
We can't move items backward in Kanban.
Of course you can! Whether or not you should will depend on what "additional work" means within your current process. In most cases, it either means:
Your process failed to complete or expose all the work needed to get a task to the Definition of Done for a given ...
This problem isn't unique to Scrum. It's quite common across the Agile methods, which call for frequent delivery of valuable, working software to customers and users. I've come across two ways to handle this situation, both of which have worked out well.
The first way would be to define the infrastructure that is needed as part of both A and B. Then, ...
(Too large for a comment)
I disagree with two of your remarks under point 1:
"The infra itself has no customer value." So, get rid of the rigid requirement that "These stories are supposed to be customer oriented and deliver value.", change it to "These stories are supposed to deliver value".
"The backlog now contains ...
You're acting as if your bottleneck is something bad and you should rid of it. This may be true, but I'd like to list other options. There are 3 approaches to dealing with constraints:
Improve their performance (Bogdan got it covered in his answer) (book)
Use them to dictate the performance of the system (same book)
Use them to pivot your business (another ...
The predecessors for Task C are:
Task A Finish-to-Start
Task B Finish-to-Finish
If Task A is ID 1 and Task B is ID 2, the Predecessor field for Task C should be 1,2FF. Note that finish-to-start relationships are the default so the FS is not needed unless there is a lag (e.g. 1FS+2d would indicate start the task 2 days after task 1 finishes).