I don't see a "why?" in your post. What do you expect to gain?
Surely if you use 4 gears on a 3*9 setup you will use a similar amount on a 1*9 or 1*11?
The gear range is what's important for me.
Honestly, I can't fathom using only 4 gears. I used more than that on my daily 20km commute in the Netherlands - not exactly the hilliest part of the world!
With respect, the mileages you have covered are of little relevance without elevation data (altitude and gradient).
My understanding is that you are planning a cross country tour? In that case you will encounter a variety of situations where due to tiredness, aches, an injury, weather or extra loading mean that you may wish to pedal outside of your usual combos.
There's also the fact of greater flexibility with front as well as rear options in the event of unexpected damage or wear and tear.
There's always a push (especially online with targeted ads) for the newest.
From my experience it's easy to list the advantages of buying something new, not always so easy to calculate the costs of losing the old.
If I was in your shoes I'd look at the cost of changing - not just the front chainrings but also the bottom bracket, the cost of components (cassettes and chains) and the likely availability of those components where I plan to travel.
I'd also look at what else I'd lose.
For me the flexibility of a 3*x setup would be a loss.
The ongoing costs would be a factor, as would the availability of replacement components on the road, especially if going with 11 sprockets.
Bikeshops are always looking to sell or upsell and some YouTube channels are sponsored and you can be pretty sure that the hosts often get much better service than the likes of you & I (I'm not familiar with the channels you mentioned).
Then again, my bike is a 3*7 (currently running with an 8 speed cassette) so what do I know?
Good luck with your decision.