Here is your problem Gable Vents and Ridge Vents do not go together period.
Your Gable vents are acting as Inlet Air Ducts for your Ridge Vents.
So your air flow comes in through each gable and flows immediately to the closest air outlet the ridge vents NEAREST the gables.
So the Air flow in the center of the Attic is basically NON-EXISTENT . And the Actual cooling from Gable to ridge vent is very limited.
Either one or the other but not both.
So here are some options:
Option 1: Close your ridge vents.
Option 2: Close your Gables and Install Soffits (I know you don't want to hear that - keep reading though) - these would need to be properly installed - a soffit guide is best - so your insulation and critters do not block them.
Option 2 will allow you to get rid of your Electric Fan as well, saving you money on the electricity of the fan. As for how much difference Ridge Vents vs. Gable Vents I can not tell you if there is a savings or not a savings, getting that info is like getting an act of congress for something that matters.
With Option 2 you have direct line air flow from outside upwards to the roof all working by convection - $aving you $$ .
Now if you do this a vent here and there might be expensive and time consuming aside from calculating how many and what size .. Instead of doing that - they make Continuous Aluminum soffit that is pre-drilled/cut with holes. You basically lay it in like aluminum siding and it is very easy to install. When I was living in FL my house there had it. This type of soffit is inexpensive, it works great and as I said easy to install. I had to replace a couple 3 foot x 6" sheets; if my memory serves me right - I think it had cost me somewhere around $8.00 - I think it was so much cheaper than what I thought I had bought a couple extra with out skipping a beat. Probably a bit more expensive than that now - I went to a local metal soffit fabricator shop and purchased it.
As is typical of your past questions, you are being hyperbolic and absurd.
ASHRHE suggests a ventilation rate of 40 to 70 CFM - if we took 70 CFM and 4 air changes per hour, your house would be a 10 foot cube, roughly. As far as I recall past mention of your purported house, it will be much, much, much larger than that.
As such, the ACH (which would still be at most 70 per that standard) will be far below 4. And the surface area of poorly insulated or uninsulated building
to enclose that area will be large, and losing a metric-butt-ton® of heat as a result.
I've run the numbers in past answers.
Are BTU's only calculated using square feet?
Dropping them into my shop's spreadsheet, 70 CFM is 0.1736 ACH and amounts to 6381 BTU/hr at design temperature (-20°F). It will be a shade less if yours is only -10°F The walls (R33 SIPs) account for 6709 BTU/hr at design temperature. If you made the walls crappy R11 they'd burn 20128 BTU/Hr instead. I don't even need to get into the ceiling, floor, doors, or windows (all additional heat flows) to make fairly clear that even without a HRV, yes, insulation matters. Of course, if you added an HRV you could shave off 5104 btu/hr at my design temperature, and that's something I'm certainly considering doing.
Best Answer
Enough heat is the first need.
Insulated sheds get cold inside, moisture condenses. Water can even freeze.
As my boss found out with his boat. Insulation just means that cold nights make the interior get progressively colder and not get any thermal gain during the daytime, unless your floor has surface area that transmits enough ground heat. And if you forgot to drain the engine block... Bad things happen despite that insulation.
Once you have heat, a little ventilation as necessary to remove moisture given that it has a good floor that prevents water wicking up into the interior space will help.