Hang in there, it takes time. Been SR shaving for less then a year and it took me quite a while to get a clean chin ( still don't occasionally ) and just started successfully doing ATG.
For irritation, work on good preshave prep and watch your touch on the razor! VERY light pressure is all that's needed. Find a good post shave balm, I recently got L'Occitane and it is awesome and soothing.

Best of luck on your learning process, it's worth it!