“PEMDAS is a convention, not a hard and fast rule.”
I’m not sure about that
In order to have meaningful symbolic math we all have to use the same conventions, thus making them hard and fast rules.
Otherwise suppose I say + means divide or that 5 means take the square root , or when unsure just take the last number convert it to a letter and reply with a bird starting with that letter
In order for math to work we all have to agree what to do first when faced with say: 1 + 2 × 3
If there is no “rule” then 9, 7 or for that matter 11, 5.4, 6.3333456 and chicken all become equally correct depending on the individual’s “convention”