is there a word to describe one who doesn't try very hard? not someone who doesn't try at all, but one who does their work without much enthusiasm or perhaps with as much gusto as others.
A word with particularly bad personal connotations is "underachiever"
Per Weather Vane, from Cambridge again: https://dictionary.cambridge.org/us/dictionary/english/underachiever
underachiever noun [ C ] us /ˌʌn·dər·əˈtʃi·vər/
a person whose performance is lower than you would expect, based on that person's ability
Someone might make lacklustre efforts.
The Cambridge dictionary gives the example
Britain's number-one tennis player gave a disappointingly lacklustre performance.