The course from peter clarke is pretty good and crushing value for the price. Didnt read the book; but in general i enjoy his style of teaching and his videos here at RIO a lot.
However: its more of a beginners course for 6max online as you mentioned.
There are some other courses around the market esp for live poker, but they are way more expensive.
Might still be a choice if you are rolled for 200 already.

For reference about my experience level, I have played micros online and 6 live cash games.
Had quite a rocky start to both but have been running pretty hot and more comfortable playing for money lately (opposed to small bar freerolls), I also took advantage of another sites free resources.
I conceptually understand things like blockers, combinations, and try to size my betting to fold out stronger hands when I am weak/ get called by weaker hands when i am strong. And i have specific strategies for targeting relatively soft 1/2 games (that would be relatively useless at higher stakes, I also try to realize when i need to play balanced )... That said even when I probably don't need to be balanced due to inattentive players I still play balanced.

I have gone through Peter Clarke's course. It's a great start if you're playing 6max, and most of what you learn will still be applicable to your full ring games, with some adjustments. That said, if you're playing live full-ring, I would steer you towards Crush Live Poker. For $400/year, you get access to multiple courses, plus over 500 videos, thousands of podcast episodes, articles, etc. Check out Bart Hanson/CLP's free stuff on YouTube to get a taste. But for my money, that's the best value out there if you're a low/mid-stakes live player. That said, if you're into solvers and intense mathematical breakdowns, etc, etc, you're at the right place. I like both, so I'm both places. Anyway, hope this helps.

Oh also, for a beginner, the Red Chip Core program is (at I think $100) a pretty nice value too. I find their stuff to be somewhere inbetween what you might find here and what you might find at Crush Live Poker.

Cheapest way to learn....

Yes. Get FTGU.

Then if you absolutely must get a book then grab his kindle book THE GRINDERS MANUAL

Get GTO WIZ ... free version is ok for starters.

Submit hands for review here and on the two plus two forums.

For llive poker you can check out Mariano and Brad Owens for free and a plethora of really good vloggers on YouTube. (And all the great live stream with pro commentary)

Regarding the table size issue 6m is 9m missing the first 3 positions. You just need to know preflop ranges is all and adjust accordingly post. GTO WIZ can help you with that.

Also wanna add that you shoukd practice grinding endurance. The true poker pros put in volume and spend a ton of money and time on mental stamina and fortitude. So think of poker training as training for a marathon not a run in the park with your dog.

Get Jered Tendlers books.

Don't think about money. Think VOLUME and TIME. You're going to play four to six hours a day and study two. . . practice with play money tables if you have to. (Live is different. I would grind two 8-12 hour days straight and then take a day to study and a day to rest - maybe not in that order. Repeat.) Maybe in the beginning youll flip the playing and study time but you get my drift...

NOW GET TO WORK ROUNDER!

Hey Ryan, good on you taking to forums like this to learn even as someone who has been playing bar freerolls mostly! I can't imagine anyone in my free bar leagues taking the game this seriously.

Without having seen Peter's course, I would assume the biggest consideration you'd need to keep in mind is that you're going to want to have increasingly tighter ranges from positions 7-9 compared to 6-max. Keep this in mind when you raise from early position and still get 3b as well. The 3b ranges in that scenario will be much different than 3b ranges in 6 max. Otherwise I think it sounds like a good starting place!