Like you, I wanted to actually learn some gun building instead of slapping some parts together.
My first was a stock blank that already had the barrel inletted and ramrod hole drilled, I highly recommend this route for starters. I assembled the other parts piece at a time until I had everything. I bought all of the how to books and watched all the building videos but hands on experience is the best and never reach for something that cuts wood faster. I did use Gunsmith of Grenville County as a desk reference but later decided that although popular there are better ways to build a gun than it implied. I would work on this gun off and on, goof up and correct my goof up, take a break and start again. I took me two years to finish this gun. I used a TOW full sized plan to guide my wood shaping.
On to #2 and more up your alley, I built a 12 ga fowler. Same as before, a stock blank from Dunlap, the barrel inletted and ramrod hole drilled by Fred Miller (now retired), a full-sized plan from Track and a pile of parts. After gun #1, I didn't need any bench reference and knew how stuff was supposed to go together. I found out on gun #1 not to follow the track plan to the letter because your parts and the ones in the plan will not be the same size or fit. On Gun #1 I measured my lock placement exactly to be the same as the plan and found out their lock was longer and had a different sear location. If I had already cut the stock for a butt plate, I would have had a 12 length of pull instead of my needed 13 1/2", luckily I hadn't cut the stock yet.
Transferring the plan to a cardboard templet for initial stock shaping.
This plan shows cut-a-ways on how the stock should be shaped from stem to stern.
I think I had a year in this one, the butt plate took me forever, I just couldn't get it gap free until I heated it red hot and gave it a few whacks where the gaps were. It would have easily been a 6 month build if I hadn't struggled with the butt plate so much. I don't work on a gun full time, there is deer season, gardening season, work-out schedule and such to make my gun building sporadic. I had the barrel jug choked full by Caywood.
Now on to # 3, a pre-carve, I have done two, never again. I didn't let them do the lock inlet on #3, THANK GOODNESS I didn't. This pre-carve was not terrible but... My blank came back with an inch of extra wood one side of the barrel and 1/8" one the other side, it was that off center. Thankfully I was building a very slender TN rifle and could make it work. Their router had gotten off track and gouged out a lot of places that I had to fix.
#4 was a Kibler SMR, it was so easy to finish I felt guilty because I only had about 10 hours in it start to finished gun.
#5 was a Track Haines pre-carve kit I picked up at a half price bargain. The pre-carve had over a dozen MAJOR flaws including a lock inlet that put the pan below the side flat of the barrel,
The wood had more figure than any I had worked with, I should have trashed it and started over with a blank but I forged ahead out of bull headedness. A year and a half later I had a finished gun and had almost no hair left on my head because I had pulled it all out when I got P$$ed at the mess I was working with.
So, go with a blank, have the barrel inletted and ramrod hole drilled, use a plan for reference and actually learn how to build a gun. It looks daunting but it is far easier to build from a blank than spend time fixing the goof ups on a pre-carve.
There is one exception, his name is Mark Weider, if you let him shape the buttstock and even inlet the lock after he inlets the barrel and drills the ramrod hole it will be as close to perfect as can be. I don't have contact info but he has a couple of videos on YouTube. He is a one man show and does stocks one at a time, the other place lines blanks up on a duplicator and hacks them into a gun looking up object.