"for the 2020 income year, on the premise"
1. Premise is singular due to the fact you used "but" if this was replaced with "and" you would have more than one premise.
2. I'm no lawyer, but I would say they could be used interchangeably here. Premise sounds better, and appears to be the better choice because you are talking about a condition of your lack of satisfaction.
3. The tax return was "for" the year 2020. This is an important designation. If you used "in" the year designated as the tax return would be vague, you would just be referring to the "one" that the individual completed in the year 2020 which in fact could be any year.