Thanks for the welcome.
Yes unfortunately I can set the U18's tactics with the staff responsibilities but not the U23's (likely a bug due to it being created midway through the save), which makes them play with the U23's manager own tactics and formation.
I have reported it to SI a couple of months ago but was looking for a workaround as there wasn't any patch regarding it and I really like developing players with the entire youth setup.
I found that going back to the club (tested this by retiring my current manager and adding a new one in another save slot), and getting the new tactics inbox and setup did fix it but this would have meant losing like 8 years or something of manager history, hence trying to leave the club and come back with the same manager.
A shame that it isn't possible but thanks for the feedback on it.