On the physical side of the abilities, Dexterity is the problem.
A conspicuous problem with Dexterity is it evolved into a mechanic that has little to do with reallife.
If you look at a reallife acrobat/gymnast, whether male or female, they are strong and buff. Always.
Oppositely, if you look at the body of a reallife renowned archer, sharpshooter, videogame champion, typist, or quilt maker, their physical agility is average. They might be potbellied, in wheelchairs, or so on, and highly unlikely to be able to leap out of the way of a Fireball, or catch themselves from a fall off a balcony.
In reallife, Dexterity has nothing to with athletics.
In formative D&D 1e under Gygax, one of the things it was trying to quantify is the difference between fine motor skills (namely manual Dexterity) versus global physical body, kinesthetic, skills (namely Strength). A person can be extremely strong, but still bad at typing.
A person who swings a sword proficiently in combat requires extreme hand-eye motor skills, balance, positioning, reflex, and so on. But one only needs to make a single Strength check, because swordfighting is wider than a ‘fine’ motor skill, and Strength represents the totality of the physical coordination of the whole body.
The funny thing about giants is, if they are too big, then dealing with all of these puny little creatures does require ‘fine’ motor skills, like typing, and they might lack manual dexterity.
Strength is the go-to ability for anything relating to jumping, climbing, lifting ones own body weight, catching oneself while falling, pulling oneself out of a trap, jumping out of the way of a trap, and so on. Strength is the check that one makes for any kind of Athletics check. Athletes are necessarily strong. Use Strength for any kind of physical stunt.
It seems to me, jumping out of the way of a Fireball, is necessarily a Strength check, using the same kind of kinesthetic athletic skill that swordfighting does. A warrior is much more likely to avoid a Fireball than a typist is. A warrior is much more likely to avoid a Fireball than a lockpick is.
Being a worldclass sharpshooter with slow steady cautious aim, is of little help versus a Fireball explosion.
Use Strength for all Reflex checks that require athletic coordination.
In D&D 1e, the confusion seems to be because of balance. And this confusion snowballed across the editions. The problem is, tightrope walking. Walking across a rope was something that occasionally happened during an adventure, and the DMs had to adjudicated it. It was obvious that tightroping had little to do with physical weightlifting. A bodybuilder was not necessarily good at tightroping. A nonathlete might be good at it. So they gave it to (manual) Dexterity. Since then ‘Balance’ checks have been used for everything ‘acrobatic’, including ‘surfing’ a Fireball explosion. This misuse of Dexterity is an error.
Only a warrior/athlete would be able to theoretically ‘surf’ an explosion. Not a typist.
Moreover, tightrope walking really is a ‘fine’ motor skill, much more focused than a total body athletic check.
Dexterity is manual Dexterity only. It is the fine motion of ones hands and fingers, like knitting. Now, if someone wanted to type on a keyboard using toes instead of fingers, it would still be a Dexterity check because it is a fine motor skill, even tho it isnt strictly ‘manual’ because it uses feet. Similarly, if someone was playing a videogame using ones feet it would be a Dexterity check.
Walking across a rope resembles playing a videogame with ones feet. It is highly focused, and requires small, precise, sensitive motion. So, tightroping really is a Dexterity check, even tho one of the more unusual uses of ‘manual’ Dexterity.
In sum, it seems to me,
• Use Strength for any kind of Reflex check that seems to require the athletics of the entire body.
• Use Dexterity for any kind of Reflex check that seems to require small, sensitive movements.