John-
Long story short. (assuming you meet development standards). I believe building anything less than 500 square ft- you will save money if you can define the project as an "addition" vs. an "ADU". Now, you may be taking advantage of the ADU code for other components- but if a project is under 500 square ft, there are no fee savings........
As for fees-
You are exactly correct. The HCD clearly identifies any ADU project under 500 square ft. as not applicable for School fees. (BTW- I think new HCD booklet is really good)
Although- my guess is this- the 2020 ADU legislation actually says, "Local Agencies" and 'Special Districts" cannot charge fees. Unfortunately, within the fee section (66000) of government code, a "school district" is determined to not be a "Local Agency" or "Special District". So school districts are leveraging that discrepancy in order to continue to apply fees.
So why did HCD write that fees are not applicable for project under 500 square ft? (That notion is not contained within any of the new legislation) Most likely, HCD is referencing the standard fee section of the government code- and it does state that a owner can add up to 500 square ft to their property and not pay "school fees".
So- you can see the problem here. Simply do an addition to a house of under 500 square ft. no school fees. But do that same project defined as an "ADU"- school districts now apply the fee.
Assuming, you have the flexibility to define your project as an "addition", and still accomplish your goals- you would be exempt from school fees, developer fees, sprinklers are still off the table etc....