Tax refund and recalculation for partners

Good afternoon! I would like to clarify which procedure for filing a tax return is optimal in our case. My official husband (married in Russia, confirmed in the Netherlands) has been living in the Netherlands since October 2017, I joined him in August 2018. My husband works for phD position (has a 30% ruling), I don’t work and don’t have any income. The question is when and what declarations should we submit, and should we do it jointly or separately? Do I understand correctly that for 2017 the husband can submit an M-form, for 2018 we should submit separately - for him the P-form, and for me the M-form, and in the future for 2019 we should submit the P-form together? Do we have the right to request a refund for 2017, since 2019 is already here? (it seems that the M-form can be sent within as many as 5 years after the requested year?) And about the P-form: if we do not have time to submit the declaration before the end of April 2019, does this mean that a refund for 2018 can no longer be requested, for example in next year? Where can I request a deferment? Can you estimate how much we need your professional help in our situation and how much it might cost to complete all these declarations? In our situation, is it even worth counting on a tax refund after recalculation?