I am looking for the strongest finish for a project with the following specifications. I prioritize scratch and wear resistance, but also want sun and water resistance along with adjustments to temperature changes. It is okay if the color changes a little, but it can't be too drastic. I will not be refinishing in the future. Sorry for so much changes, I think I am discovering more of what I am looking for as I am learning.
I want the most durable overall.
Not spar varnish then. Spar varnishes are actually relatively soft, and thus are prone to marring from abrasion that a typical indoor polyurethane would shrug off. They're also significantly more yellow than regular varnish in case you were unaware.
You want to look into marine epoxy coatings. Extremely strong in almost every way. But be warned, they are not cheap.
Depends on the conditions.
Spar varnish is typically not as hard as poly, so it isn't as resistant to mechanical wear. However that also means it handles expanding/contracting wood better, and thus is a better choice for wood exposed to weather. Also, spar varnish often contains ingredients to block ultraviolet light, which can help wood retain its color when exposed to sunlight; again that's more important for outdoor wood.
Spar varnish will usually add some yellow tones to the wood. Oil-based poly will do that too. Water-based poly typically dries transparent.
You can certainly use spar varnish indoors, if you like the look. I used it most recently on some window trim, which will get some direct sun and occasional raindrop. For most purposes it's a perfectly reasonable choice... but poly may be a bit cheaper, may go on easier, may clean up easier (if water based) and may change the color less (if water based); that last may or may not be an advantage.
Pick your trade-offs.