My road bike seems slightly too big for me After riding for about 300km, I'm looking at adjusting some parts to fit me.

First adjustment:
Because I feel that I'm stretching a little too forward, causing my back to ache, I've decided I would decrease my stem from 110 to 90.

Second adjustment:
Because I feel that the saddle is too small for me, causing the sensitive area to compress, I've decided to change my seat to swork power (OEM).

Right now I'm looking at my leg reach to the pedals. Currently I feel that I have to stretch a bit more than I feel comfortable to, while pedaling, so I'm considering if I should shorten my seat post or change my crank length from 172.5 to 165. HOWEVER my seat post is integrated to the frame, I feel like I shouldn't cut it (plus its risky as it is carbon).

165 cranks are rare: I only found Dura-Ace 7950 that would cost me $275, but it would be a upgrade from the current Ultegra.
Also my friend is saying that changing my Ultegra BB to a Dura Ace BB would make my ride much smoother.

Please advise me if all these changes I'm doing are advisable. Also crank change vs seat tube cutting!!

Thank you for your reading time!
Because I feel that I'm stretching a little too forward, causing my back to ache, I've decided I would decrease my stem from 110 to 90.


You can help this by also moving your saddle forward.  If you combine that with raising your handlebars some by changing the stem angle, you'll shift your torso to be more upright, too.  That will also shift your weight back a bit, putting a bit less pressure on your hands and a bit more on your bottom.

Moving the saddle forward would also slightly shorten the reach to the pedals.


  165 cranks are rare


They sure don't seem to be.  A simple Google search returns many examples of Shimano 165 mm cranksets that would likely all be compatible with your bicycle.


  Also my friend is saying that changing my Ultegra BB to a Dura Ace BB would make my ride much smoother.


Hogwash.  You wouldn't be able to tell.  You'd probably be surprised at how many amateur road racers use things like Sora or Tiagra cranksets, or won't bother paying more for Ultegra - much less DuraAce - over 105 or today's Tiagra.  (Wheels are what gets the money - good aero wheels can save 40 or 50 watts - maybe more - at 30+ mph...)  Pros and higher-level amateurs ride high-end bikes because they get them for free - it's marketing aimed at getting you to pay a premium for bling.  DuraAce doesn't make your bike faster.  All it really does is make your bike a little bit lighter and a lot more expensive.  How much better is a $275 DuraAce crankset than a $65 Sora crankset anyway?  IMO, don't worry about the grams until you get down to 2% body fat.