About to pull the trigger and I keep debating on the amp portion. Not to gloss over it effectively doubles the price of the audio system, I'm wondering if anyone has any first-hand accounts of comparing JUST the speaker replacement vs speaker replacement AND the amplifier? (I am not planning to do the subwoofer).

More than anything, while the installation video seems straightforward I am mostly a bit overwhelmed thinking about doing all the steps and removing everything without damaging or breaking something..

And, of course, does having the amp basically making the sound replacement 2x better?

If the speaker replacement is 2x better over stock, than is having the amp 4x better?

If you’re replacing the head unit and speakers, you should also replace the amp. Even the factory JBL amp isn’t very good. It all depends how important sound quality is to you.

Not using any of the factory wiring, for me, made the installation relatively simple. I’ve done three full system installs in three different cars over the years.

You should definitely replace the subwoofer also. I bought a Pioneer 8” shallow depth subwoofer that fits in the factory subwoofer location. I think I paid less than $100 for it, and it sounds a lot better than the factory subwoofer.

A good amp goes a long way. Look at the Watt RMS rating and compare it to your new speakers, this tells you the most about how much power it will send to the speaker and match it to the speakers. Let's say your new speakers can handle a max of 50 RMS. You'll want a amp that has a 50RMS x 4 or 50RMS x 6 or however many speakers you have (count component speakers as one unit even though the tweeter is separated). If you install those same speakers into the stock system with a head unit that only puts out 20RMS x 4 or 6 it won't sound much different at all from your stock unit and can actually sound worse when played at higher volumes.

One option is to replace the speakers now, see how it sounds, then consider adding an amp. The work required to swap speakers is independent of adding an amp, so it's not wasted effort to do it in two phases.
If you do decide to add an amp but you are going to stick with the factory head unit, you'll need a line output converter (converts the speaker-level outputs from the factory head unit to line-level for input into the amp) or buy an amp that can accept either line-level or speaker level inputs. Some amps do, most amps don't. If you replace the head unit, it will have line-level outputs that plug into any amp. If you decide to add an amp, the amount of work goes up, though. But if you finish the speakers, maybe you'll realize that it's not too big of a deal to do the amp.
I can't say about improvements of 2x or 4x, but better speakers are more efficient (louder) and will probably sound better. Adding an amp (with reasonable power) will make the new speakers come alive and will be even louder.
I've got a pile of boxes in my living room for a complete stereo replacement, but it's many thousands of dollars worth of speakers, amps, subwoofer, head unit, wiring and sound deadening material, so mine should be in the 10-20x better range!
Good luck!