Income tax paperwork related to your job would surely just be payslips? As your anonymous I've no idea where you are from but in the UK you get a tax summary page when you file a tax return, if you get similar try that
Thomas Tohnan passive income could help to show how you can support yourself if asked during a soft power application. If you manage your rental properties remotely you could use this as the basis for the workation option but if someone else does this you can't
Of course it makes a difference which option you check. You apply for the DTV for a purpose and it is issued to you for that purpose. If you intend to work remotely from Thailand then you need to use the workation option. If you won't work but instead will follow one of the soft power options then you apply for soft power
You apply for the DTV reason for a purpose, and you are issued the visa for that purpose. Just like any other multi purpose thai visa such as the Non-O