My understanding of the utility of Greek letters in notations was their correspondence to first Roman letter of the respective concepts being represented [1].

Just as Roman letters could be stand-ins for concepts in the context, say, v for volume or speed (vitesse), R for resistence, L for length, d for diameter, r for radius etc.

So, for example, quantity /variable of [l]ength could be represented by [l]ambda, [a]ngle by [a]lpha, [d]ifference by [d]elta, [s]um by [s]igma, [e]rror by [e]psilon, [ph]ase by [ph]i etc. Yet [e]fficiency is eta (not epsilon), maybe it's due to the [h]eat energy context. Perhaps azimu[th] by [th]eta, as in spherical coord (r, phi, theta) or maybe zeni[th]...

Not sure why density was expressed with rho or why pi for circle's L/d ... Maybe it was just a natural choice in Greek?

Of course, there is no 1:1 correspondence, it all depends on context and anyone is free to choose anything, especially where it does not need to mean anything, simply a stand-in.

So not feeling like using a,b,c or 1st, 2nd, 3rd? How about alpha, beta, gamma? Here's a handy table [2].

Btw, omicron is 15th.

[1]: https://en.wikipedia.org/wiki/Greek_letters_used_in_mathemat...

[2]: https://www.rapidtables.com/math/symbols/greek_alphabet.html