Upgrading parts on my 2007 Trek 3700

So I bought this 3700 new when it came out, for more road cycling. The closest I get to MTB is gravel roads. Despite being what I’d consider, a more hard-core cyclist, I know pretty much nothing about bikes. I used to do around 3-4k a year, but now that I’m touring, its been closer to 8k the past 4 years. Only in the last few years have learned some very 101 basic maintenence. How this bike has survived my prolonged neglect with my mileage is pretty amazing. Now that I’ve done 2 RAGBRAI’s and currently training for my 3rd, I WAS looking at buying an actual road bike capable of touring, but now learning how the state of the cycle industry is wildly overpriced, I don’t want to change bikes seeing as it seems a 2k bike will get me something similar, but with disc breaks that I could get with a Walmart bike. But even the thought of replacing has my heart torn because I’m extremely attached to this bike. And with that bit of history out of the way, my questions based on recent research.

Front fork- So upon learning that my hard tail fork actually lowers the power transfer of my pedaling, I want to get a rigid fork. Is this actually worth it? If so, what specs am I looking for? Looking at 3700 specs, it’s all honestly confusing to me because I can’t even find the color of my Trek, as sites say there were only a couple colors, but mine this army green and brown. (I bought it solely because I liked the color…I was a teenager).

Size- Speaking of buying on color alone, I know now this bike is too big for me, it’s 19.5″ and I’ve been told in the past I should be an 18″? What can I do to make my ride more comfortable? Granted, I’m very used to this bike so I have no idea what upgrades could pertain to this.

Gearing- I often ride on the hardest setting (I don’t know the term). Nearly always between 3-5, and 3-7. Very rarely do I need to shift below that. I know nothing about gear…well gear. I only recently learned what the words cassette and crankshaft are. I’ve only replaced my cassette a couple times in its life and I didn’t even know there was an ideal schedule maintenence wise. My crankshaft is damaged, being able to not use the 2 setting (which never bothered me because I never needed it). So what options are there to give me more options when the hardest is already my baseline?

I have trusted my bike rural bike mechanic for years, so there have already been upgrades at his reccomendation (derailer, hubs, got double walled rims, etc). But he retired so I’m on my own. Currently waiting on a Brooks B17 seat to arrive in the mail because i had no idea the seat matters too.
And because I’ve shown myself to be an absolutely newbie to the bike world tech wise, any other recommendations to give me a more ideal ride?

(This is pic is on my ride home from RAGBRAI last year, riding a total of 1k. This year I plan to ride to and from RAGBRAI, which will be around 1.5-1.7K as I’m still figuring my route. For anyone curious, I built my rear pannier rack, based off more expensive racks I saw online.)

by steamshovelupdahooha