Apologies in advance for my tone, I'm not being mean, I'm just really direct and think you need a shake, so here goes.
Fear in the context of minimal risk is an idiotic motivator.
What are you even afraid of?
Seriously, what are you actually afraid of?
Is your only fear walking away from a marginal amount of income that you don't actually need?
Let's put it this way, would you start working Saturdays just to make marginally more money? If no, then you have absolutely no logical reason to continue working Fridays.
Is your life already perfectly balanced?
Do you already do EVERYTHING that you want to be able to do in every given week?
Do you already have enough time for:
-Exercise
-Cooking
-Friends
-Family
-Reading
-Quality time with kids
-Learning new skills
-Sex
-Meditation
-Giving attention to pets
-Cleaning/organizing/maintaining your house
-etc, etc/ whatever you wish you could do???
I'm going to go ahead and assume NO, because I downshifted a lot more than 20% and still don't have the time to do everything in life I want to do. The list of languages I wish I could learn is longer than I can manage in a single lifetime.
As someone who has dropped incrementally from 100hrs/week to 10hrs/week over several years, each drop felt intimidating at first, but after just a few weeks, it just felt normal and my free time filled with just as interesting things.
I just learned to ignore the stupid agonizing. It's a human instinct, but it's about as useful as being petrified of spiders.
Truthfully, I've never really noticed the drop in income. The human brain just kind of adjusts to whatever the new normal is. It doesn't really feel like a loss. I can intellectually recognize it, but I adapt pretty quickly to getting what I get and move on.
Also, I've really come to focus on my pay in hourly or daily terms. Why would I work an additional day if I have to take a tax-based pay cut to do it? Why would I undervalue my precious time that way??? I'd literally rather spend my Friday doing volunteer work than go back to the office and do the same thing I did on Thursday, but make less money.
The world is fucking FILLED with incredible things to do, try, learn, etc. Are you really, REALLY going to sell yourself short, working an extra day or two each week at a pay cut just because you can't think creatively enough about what you could be doing with your time???
Can you really not think of anything better to do with your Fridays than to go to work for less pay? If that's the case, you might as well sign up for Saturdays, and maybe even Sundays as well.
I doubt you're even afraid of cutting back, I'm guessing this is more run-of-the-mill fear of change, which is SOOOOOOO BORING, and honestly, so done to death.
Come on, you're a Mustachian, it's all about voluntary discomfort and disproportionate benefits.
If you can't bring yourself to break the inertia over a blatantly obvious, virtually zero risk, essentially guaranteed to improve your life change, THAT YOU ALREADY KNOW YOU WANT TO DO...well...then what else in life are you missing out on because you aren't willing to challenge your comfort zone???
Seriously, think about that. Think about every single possibility in your life that you might be denying yourself out of fear of the unknown.
Staying at your job for 5 days a week even if you don't need to financially just because you like the work is like eating the exact same meal for dinner every day and never trying anything else because you like that meal enough.
Btw, I absolutely love my work. Like, not enjoy, but capital "L" Love it, and it makes me feel alive. I have had to downshift for medical reasons. I trained for 11 years to do this, which cost me a few hundred thousand dollars.
I definitely did not want to downshift, but even then, it hasn't been a bad thing. There are things I enjoy just as much as my job. There are projects I've been just as passionate about. And most importantly, I've really thrived from branching out and diversifying my life, challenging my own expectations, and getting extremely comfortable with messing up my comfort zone.
So if someone like me who LOVES their job, only got to do it full time for 3 years, and didn't get even close to FI, can embrace downshifting and be grateful for the amazing opportunities it opens up for me, then I think you can handle maybe taking Fridays off.
Again, not being mean, this is a truly friendly "what the fuck is wrong with you?" shoulder shake from someone who thinks you are missing out on your own life.