Is there a word that works as a milder version of strenuous? Strenuous is defined by Oxford as something "requiring or using great effort or exertion." I'm looking for a word with a similar definition except for the "great" part.
characterized by steady, earnest, and energetic effort
It's hard to define the degree of effort implied by a lot of these words, but you could try something like tiring:
Definition & Example (via Merriam Webster):
causing weariness, restlessness, or lack of interest
the seminar was tiring and not particularly helpful or informative
Maybe difficult will do:
Needing much effort or skill to accomplish, deal with, or understand:
'she had a difficult decision to make'
'the questions are too difficult for the children'
I would suggest taxing:
Physically or mentally demanding.
Or even laborious:
(Especially of a task, process, or journey) requiring considerable effort and time.