You can hit the consecutive positionals on DRG while using jumps that lock you for the better half of your GCD so it's entirely possible on MNK aswell. If you're standing at the right spot, switching between rear and flank is only a tiny step.
Bosses will occasionally turn, move or get pulled somewhere else and preparing or reacting accordingly simply requires fight knowledge just as a DRG or NIN needs to know when it's safe to get animation locked, a BLM when they will have to move etc. and if you happen to miss a positional... so be it. It's not like other classes can play 100% at peak efficiency either. They all have things they need to account for, for MNK it's more positionals.
Not every class has to be for everyone but if you really want to learn it, asking for it to be made easier isn't the way to go. "Make it easier" really shouldn't be your first instinct when there's other things you can do and not every class has to be equally accessible for everyone.

I second practicing on a dummy. It immensly helps to get the skills down to muscle memory which makes it a lot easier to react to mechanics.
Play around with differend keybindings to see which feels most comfortable to you. Frequently used skills need to be easy to reach. Same with skills that are part of a very busy phase even if you don't use them every few GCDs.
It may take several sessions with a dummy and a couple of hours until you really figured everything out but that's not unique to MNK. It takes time to find a layout that works for and that particular class. Adjusting some UI elements may help aswell. Something many players do while learning a class is making an extra hotbar scaled bigger than the rest with cooldowns that they place somwhere centered so they don't forget their oGCDs while still trying to get the general rotation down.
If you notice certain mistakes happening frequently, see if you can adjust your keybinds and UI to make avoiding them easier.