You're getting dimensional agility at L3. You don't obviously have a means of teleporting at that time, so it's not normally available. You can retrain your L3 feat to dimensional agility once you get abundant step at L10 of course, but some people see that as dubious, and in any case you need to note that you're doing it.
Assuming that every punch hits is a bit much. +28 vs the mean CR 20 AC of 36 is still missing 35% of the time, and concealment or other non-AC defences are fairly likely on CR 20 enemies. You have no means to beat concealment, invisibility or similar.
There are plenty of ways to get enlarge person without needing a custom magic item. By L20 it should be trivial to have the use magic device skill required to use a wand on yourself, or potions are easy enough. The main problem is your spell resistance which is a problem with any of these including the custom item. You may prefer to use qinggong monk to trade this ability away.
Your AC isn't enough to take on CR 20 creatures (AC 32 vs ~ +27 means you get hit a lot, AC 37 w/barkskin is still a fair few hits) and you have no non-AC defences.
The traditional Monk have BAB of 15/10/5 and Flurry of +18/+18/+13/+13/+8/+8/+3, and i use this as base to calculate the BAB flurry of Monk Unchained
Sorry, not quite. Unchained monk was meant to be one of the various monk fixes, this time trying to shake up the original formula. So it is always best to go to the new text itself, rather than relying upon previous knowledge.
Anyway, the unchained monk threw out the pseudo TWF style previously used, and it just gave you straight bonus attacks at full BAB. I believe the attack routine is +20/+20/+20/+15/+10/+15 (before ki or haste).
Sidenote- could I interest you in the original monk using the sohei archetype? It grabs weapon training (and could potentially use gloves of dueling), leaving its attack routine at +23/+23/+18/+18/etc. If you grab pummeling style or outslug style, you can get fairly easy flurries. Weapon training adds to your damage, and you can also grab brawling armor (sohei can flurry in light armor; not the best for AC, but +2 attack/damage...).
some pointers:
get a ring of ki mastery to decrease all ki uses by 1 (minimum of 1). it used to cost 10,000 gp but seem like it was errata down to 5,000!
if you are not good aligned see if you can pick ki leech qigong ability as well. these two will make sure you are never lacking ki.
if going for jabbing style (which i totally would) i would advice picking the 'Elbow Smash' style strike instead one of the others as this is effectively an extra attack at -5 if you hit so a lot more damage (nonlethal but still great. also see enforcer below)
i would change the whole Dimensional tree -which i believe was to get the enemy flat footed with savant, and as pointed out ain't working. you need flat footed not flanked. (ur mixing them i think from sneak attack that work on ether flat footed OR flanked. medusa's wrath doesn't work with flanked).
instead i would work on your intimidate skill (maybe add items that buff it if able). go for the
weapon focus(unarmed strike)
>dazzling display
>shatter defenses - now shaken enemies are flat footed so medusa can pop up
+ enforcer (for that elbow smash non-lethal).
taking the scaled fist archtype will not only help with intimidate (as they use cha instead of wis) but also give you access to shatter defenses feat at level 10\14\18 bonus feat without the need for it's per-requists. so you only need enforcer beside it to pull this off.
attck wise you go with closing in with flying kick+stuning fist to increase chances of making target open for medusa) if not already in melee range, then elbow smash +the extra attack, if that one hit intimidate and get 2 medusa strike attacks.
can also invest in other things to make enemy fearful
when in melee range just 2 elbow smash.
when hasted at level 20 with 2 elbow smash and medusa and ki point for extra attacks asumming all hit would like like so. (can add stuning to elbow smash for extra chance of getting medusa's wrath)
bab :+20 (elbow)/ +15 (elbow,non lethal +intimidate)/+20 (medusa)/+20 (medusa)/+20 (elbow,1st extra flurry)/+15 (elbow,non lethal)/+20 (2nd extra flurry)/+20 (haste extra)/+20 (ki extra)/+15/+10/+5 = total of 12 attacks for jabbing master to work with!
2nd hit add 2d6 from jabbing master and every hit after adding +4d6 from jabbing master.
also remember , if elbow smash miss the nonlethal attack, or if u need to use other style attacks, you can always deal nonlethal damage with ur unarmed strike without any penalty so you can proc enforcer with .