I would put in a few points.

1) While its stun is on the GCD, it does have several things that aren't on it including a lot of defensive cooldowns that really should be used frequently and a silence/hard hitting attack. PLD does have a simple set of attacks compared to other classes, though they will be getting more in Heavensward, however its thing really is using its cooldowns smartly. The slower GCD also allows for managing more complex mechanics. If you've ever watch a video of Titan Extreme you might see what I'm refering to.

2) I think part of the reason you have a lot of grouped content is because the scale of what you do needs to realistically match what you can do. The WoL on their own attacking into the heart of Garlemald power in Eorzea and killing their generals kind of belittles them as enemies. Leading a team of elite fighters fits the narrative better. Likewise is true for the primals. We are supposed to be great warriors not gods in human form. It also allows for complexity to those story encounters.

3) The issue with dungeon difficulty in my experience depends a lot on the group you get. Sunken Temple of Quan was very easy my first run but Stone Vigil was a nightmare. My group wiped about 5 times on the second boss. Do remember a lot of these dungeons you will be doing with much higher geared players who are downleveled.

4) You comments on the last two 8 man instances are both valid and both come up a lot. The core problem with those two instances is having the Cutscenes in the middle of the experience thus being ruined by speed runners. Fortunately SE has avoided this in other content. Again I point to my first comment in relation to making it soloable. If the Garlemald empire can have a sole individual waltz into its most secure base and wipe out its leadership then it belittles them as a threat and portrays us in a light that make any further enemies we encounter afterward difficult to realistically portray as a threat without this constant villain power creep of silly proportions.

5) RMT is definitely a problem for US servers. Strangely they seem almost non existent on Japanese servers. Its something they do need to work on.

6) They did mention they are making some improvements to the Chocobo companion as far as queuing for things with it out. I doubt you'll see a change in the levelling curve though. I think they want it to be something that takes a long time as a maxed out companion is quite a powerful asset. Also it would kind of negate the whole value of FC chocobo stables and gardening to farm chocobo feed.

I'm glad you are enjoying the game and I agree there is plenty of room for improvement. I would suggest at least watching a few video guides on the Hard Mode and Extreme Mode trials as it might give you some insight into end game combat and the value of that slower GCD. Also try out Crystal tower if you haven't already. Its pretty straight forward and a good way to get gear.