What's the difference between etymology and glyph origin?
Tl;dr: A possible rule of thumb is by gauging whether the query is of the nature of a word, or a character. But the correct use of tags indeed depends on the knowledge of the asker. The dependence of correct tagging on knowledge is sometimes unavoidable.

I think it is very important to understand how the tags etymology and glyph-origin interact with established concepts such as 形音義 (lit. form, sound, and meaning) and 六書 (liushu, lit. six scripts). To me, in summary:





etymology
glyph-origin




Unit of query
A word, i.e. ≥1 character(s)
A character


Emphasis of query
Evolution of meaning
Relation between form, (sound), and meaning


Liushu involved
Rebus characters, derived cognates
All




I will use the term 'sense' interchangeably with 'meaning' below.

Idioms (e.g. 老驥伏櫪), loan words (e.g. 幽默), and binomes (or lianmian, e.g. 逍遙) go well with etymology. The asker is interested to know why the word means such, usually implying they tried to parse the word with the knowledge of individual characters, but failed. Now this may be due to classical Chinese, or the simple fact that the word is unparsable. That is to say, the threshold for successful understanding is high in these words. Conversely, even if the word is easily parsable to the average learner of Chinese, they may still be interested in understanding the history of the word, particularly polysemous words.
Of course, an elementary learner of Chinese may fail to grasp the meaning of 快樂 due to incorrect parsing, compared to say a more advanced learner who already knows 快 does not mean 'fast' here (perhaps after exposure to words e.g. 快意 and 快感). I think it does not hurt to use etymology, where answerers will then expound on the various meanings of the classical 快 (perhaps their relationship too, and which sense came about first, if evidence exists). Of course, using meaning concurrently is preferred.

Frequently, when using glyph-origin, askers are motivated by wanting to understand how the form of a character they see relates to the meaning they presumably already looked up. How is the form of 果 related to its meaning 'fruit'? Answerers will then explain the character is a pictograph of a tree bearing plentiful fruits. For other characters, the liushu involved may be different.
Of course, if the asker incorrectly thinks there is a direct
relationship between the derived sense (引伸義) 'result' (as in 果然,
結果) with the character form and applies only glyph-origin,
then editors should add etymology to the question too, because
this is a complex question that asks

What is the relationship between the senses 'result' and 'fruit' in 果? (etymology); and
What is the relationship between the sense 'fruit' (not 'result') and the form 果? (glyph-origin)

To the first half of the question, Buddhist application of 果 can be cited (the translation of phala 'fruit' in Sanskrit into Chinese, itself meaning 'fruition' or 'effect'). The historicity therefore inclines to etymology.

There are words that comprise only one character, which defeats the rule of thumb. They are usually polysemous too. Let us use the fairly classical 鬻 yù meaning '(v.) to sell' or '(n.) gruel' as example. One may ask:

What is the relationship between the senses '(v.) to sell' and '(n.) gruel' in 鬻?

and tag it under etymology, incorrectly thinking there is perhaps a somewhat obscure relationship between the two senses. Or

For the form 鬻, how did it develop its meaning?

They do meet superficially the criterion of the tag, but in actuality, glyphic knowledge, specifically rebus in liushu (假借), is required.
'(n.) gruel' is the only sense that is explainable from the glyph. Paleographers believe the form 鬻 is borrowed to mean '(v.) to sell' out of phonological similarity (yù) to a (perhaps characterless) entity that means '(v.) to sell'. Gradually the sense '(n.) gruel' faded and '(v.) to sell' became dominant.
In that case, editors should add glyph-origin to the question.

Sound is often not the concern of askers, but when asked, questions like

Why is the form 鬻 pronounced like yù?
Is 粥 the phonetic component of 鬻?
How is the sound of 鬻 reconstructed in Old and Middle Chinese?

should be tagged under glyph-origin, perhaps alongside with other tags. That is because reconstructors educatedly compare characters related by sound, e.g. 粥 and 育, and make reference to liushu as well, alongside other phonological references e.g. rhyme books.