To take advantage of the current "known" entry conditions, you will need to gain Residencia this year (the pre-requisites are health insurance, regular stream of income to your Spanish bank, padron, NIE, house deeds/lease etc., proof of having paid the Residencia fees at a bank).
The qualifying requirements are expected to increase after end 2020 to be in line with those applied to people entering from other non-EU countries, i.e. those countries who do not contribute to EU finances, who do not accept EU residents into their countries without stringent processes, who do not collaborate educationally, who are not inside the EU trading/free movement zone, etc. That list goes on and on.
The qualifying requirements after end 2020 are not absolutely clear, but a much higher income level seems likely. In addition, there are no guarantees that the forthcoming (?) EU/UK deal will fund healthcare for Brits in Spain who gain Residencia after 2020.
Those of us Brits who already hold Residencia in Spain recognise that we are privileged. Brexit undercuts that privilege for those who aspire to follow us.