Welcome back! One reason I'm far from a gametime dunk, b/c as a left hand player with a RL plant, im 90% likely to be facing a defender who is well prepared to neutralise my drop stop dunk attempts. So on the right block where I like to post up, if I drop step to the rim unless ive bodied the defender pretty good, it's just easier to slip in a layup or another graceful type move rather than power. OTOH I can spin better on a drive from the left block but then im out of luck for a RL plant with a L finish. lol. Just mix of factors multiply the degree of difficulty. Can 100% your observations on fast break dunks. My spin on the R block is slowwwwwwwww af tho but I can still beat the defender who doesn't know how to defend a good left handed post player which is suprisingly common. I actually have great post presense for an undersized PF. I can score at will on the R block. If the defender is a lot bigger than me then i'm beating him with speed. If he's smaller, im going thru him no probs.
In future i expect two things can help my dunking ability in games. Just being bigger and stronger and overwhelming the defender with power, getting them out of position then drop stepping them. Will be there in 345-15 days 
As far as the drop step goes I'm the same as you (actually opposite RL-RH which is the same) but one advantage I have is that my spin move is much much better on left block so it's a bit more natural... if you LRL and spin better on left block then you truly are the worst of all possible combinations lol. Ouch.
Two things to work on other than getting a faster spin to avoid the defender completely.
Work on a drop step where you don't actually face up completely. This requires the dunk to have a bit of turn, so it's a drop step spin and the dunk has about a 80 degree spin in it. This allows you to protect the ball and even use your lead hand to fight off the defender (and even push off slightly for a boost). If you have a trainer parter it's best to practice w them - practice your spin, gather, protect and push off w your lead arm on their shoulder... hard to get this perfect but when you do you can get truly nasty dunks...
The other thing to work on (now that I have given up learning opposite plant) is using your non dominant hand... for you if you can start integrating your right hand you will be shocked how easy you can get right handed dunks on the right baseline... not only is the ball protected but you can dunk w the ball behind your body which is sooooo easy... it's a world of difference. You can keep soooo much more speed in your jump because unlike LRR or RLL dunks where you have to dunk the ball "down" to not hit the back rim and miss the dunk you can throw it in at a much flatter angle... I'm no lefty and my father sucks but it's still far easier for me.
I'm assuming that since you are a natural lefty that like most left handed people you probably have a good degree of ambidextrousness and would take to this well...
It's funny how in game dunking is so opposite of perceived difficulty... people rate my dunks as more difficult and impressive when I do left handed, two handed, or drop step rotating dunks... when really the first dunk I ever did was two handed and the "easiest" dunk (all alone single handed right hand dunk) is actually still damn hard to pull off....
