No reason you can’t start tentative planning now. OK- a couple of special notes. RS may add 1 or 2 new tours each year, in 2021 they may drop some dates or locations depending upon CV19 status. But ignoring those issues just use the 2020 schedule; just don’t buy airline tickets or book extra hotel nights until after the tour is formally announced and booked.
They really have a set system. Look at 2020 tour dates and you will see the pattern. XYZ tour always starts on Monday and Wednesdays. So if that tour starts on every Monday and Wednesday in September and October of 2020 it will most likely do the same in 2021- the date will change by a day or two (9th to the 11th or whatever the second Tuesday of the month is in 2021). The “day of the week” for the start is the controlling factor. RS tours hit a lot of Museums, sites etc and they have to work around Museum closed days. Which thus controls start days. And if you see no tours listed for the last week of June through the 2nd week of August for a particular tour you can assume the same period for 2021- usually not held because of summer heat. Again, I wouldn’t pre-spend money on nonrefundable deposits but you can easily plan now on a window shopping basis.