Need bike upgrade advice

I’m in search of my second road bike, as I have been riding for almost 3 years now, and I’m looking to change out of the bike I currently have

I ride a 2013 Cannondale caad 8 58cm with shimano 105

I am 6ft even, 155lbs, 31.5inch inseam

And about a week ago I found out i probably should be on a 56cm bike, which I do feel a little stretched out on the 58

I’ve found a few facebook ads for roubaix sl8s that are within my budget (under or around $2000) but my current caad 8 weighs 20lb, and so do the roubaixs, would this be considered an upgrade even if it’s the same weight but I’m switching from aluminum to carbon?

Other questions; I found this tarmac s works sl4 frameset for sale at a decent price (the park tool stand is included) but I hear that it’s risky to buy old carbon fiber, is 2013-2014 (the years of sl4) too old?

Okay lastly. Would it be wise just to get either a used specialized Allez or even a brand new one and build it out with carbon wheels and other upgrades? Or is that useless just because it’s aluminum.

As you can tell I’m a little scatter brained and just want my next bike to be a good decision and last plenty of years, so feel free to critique me on anything, I’m all ears and won’t be offended.

Most of my riding is 20-50 miles, around 18 mph, and I did my first time trial race this summer but would like to maybe do more race events in the future once my skills improve (the engine is the best upgrade!)

Thank you for listening to my rant