Hi,
Aperture is the diameter size of the physical opening into the optic. This diameter size determines resolution potential. Larger apertures (physically larger diameter openings) have more resolution potential. Larger apertures are going to be more sensitive to poor atmospheric seeing, they resolve more, and they resolve bad seeing just as well as good signal from a subject. This is why when you reduced your aperture on your lens to 18mm, it was "sharper." You threw away the system's resolution potential and it no longer was resolving the poor seeing from the signal and you get a steady low resolution image as a result, but it will lack any sort of fine detail on the subject and if sharpened will just be a bunch of bulbous artifacts on a disc circle.
Aperture size has nothing to do with "filling your camera's sensor." What fills your camera's sensor is the focal length relative to the sensor size.
Taking hundreds of single shots in blurry seeing conditions will give you a consistent result of poor seeing. This is not validation.
Exposure time of 10ms (1/100th of a second) or less is ideal for freezing the seeing. You generally need to take many hundreds to thousands of sequential images (ie, video at high FPS) to beat the seeing (lucky imaging). F11~F50 is not a problem in solar, there's plenty of signal, your exposure time should still be short. If it's not, then your filters are blocking too much signal and it's ok to use some gain/ISO as needed to fill your histogram.
When you close down the iris in a camera lens, the blades mask the light cone which effectively reduces aperture. Because the focal length is constant and the aperture just reduced, the resulting focal-ratio changes. This is not important until you get into sampling, where you relate wavelength to pixel pitch size to focal-ratio to sample resolution. This is not a concern for your needs, you will virtually always undersample and that is fine for this purpose.
Instead of comparing your system through poor seeing conditions, instead, use your system on a terrestrial subject without your solar filters. Just camera + lens and camera + telescope and look at a distant object terrestrially like the top of a light pole or building windows, etc. Something with detail. Compare the results of your lenses how you intend to use them to your 72mm refractor and expose them the same, then simply look at the differences on the images. They shouldn't be that dissimilar, but there will be some local heat (seeing) effects if shooting over roads or buildings, etc. Still it should give you an idea of resolution and contrast differences and your ability to fine focus either one for a sharp in focus image. Do this on the same subject and share those images here if you wish.
It's very common for people new to imaging in daytime in poor seeing to have problems with even small telescopes due to poor seeing conditions. 3~4 arc-second seeing conditions, or worse, are no joke. You can't even get a focused disc limb in that without patience and if your focus is even slightly off, the whole thing is quadratic function blurred with poor seeing in addition to out of focus. Larger physical apertures are more difficult as they resolve this even better. It even happens with smaller 60~80mm aperture refractors, let alone 200mm apertures where people immediately think they'll use their trusty C8's only to see nothing different than the resolution of a 50mm finder scope or worse. Seeing is everything.
If you want to improve:
Practice solar imaging now with the gear you intend to use. Don't go back and forth on several things comparing seeing condition results. Single shots, 100 of them, is a grain of sand compared to what you should be doing for daytime lucky imaging through poor seeing. It takes hundreds, thousands, via high speed video to really lucky image in poor seeing in a short time frame. If you're comparing single shots, you're just seeing poor seeing in each one. And yes you will absolutely get 100 back to back bad seeing shots easily in daytime seeing conditions.
Local seeing conditions matter too. If you're doing this on pavement, driveway, street, parking lot, etc, anything that holds heat, this is bad. If you're shooting over engine blocks, roof tops, etc, this is bad. Set up in some grass or wood deck, avoid shooting over things that reflect and release heat all day. Shooting "high in the sky" is not ideal here, that's not when your seeing is best. Seeing is best morning and evening. But eclipses happen when they happen, not at the best times of seeing, so prepare for poor seeing and that time of day the eclipse will happen for your area and get used to that time of day so you can get consistent practice in that kind of seeing conditions that you will contend with.
My suggestion since this is eclipse oriented:
Use whatever lens/refractor gives you the ideal field of view you want on your camera.
Get it focused as closely as you can.
Set your histogram exposures (brackets if you need to) with short enough exposure times to freeze seeing conditions. Do not be shy with gain/ISO.
Take as many sequential images as you can back to back through the event.
And don't waste time on the gear and miss looking at the eclipse with your eyes during totality. "Getting the shot" is not worth it missing the primordial experience, unless you're a seasoned eclipse imager/chaser.
This should be basically automated so you can hit a button and ignore it and watch the eclipse and then review data later and if you get lucky, great.
Very best,