That is, in order to get a reward in BITCOIN RHODIUM coins, I just need to buy coins and wait for the end of the next stage?
Yea, sort of, buying and holding now wont get you anything for this round but anything you hold and don't move from Jan 8, 2020 will count toward the April 1, 2020 dividend.
What percentage profit will I get for 1 quarter?
I like your project, but I want to understand the potential return on my investment.
It depends on how much is HODLed, IIRC the last round had it at about 6% annually, this round has twice the pay out pool so if there is the same amount HODLed it would be about 12% annually, there was a rumour that there might be a permanent increase to the pay out pool being announced sometime in the next couple of months but IDK, rumours ¯\_(ツ)_/¯
(not my project BTW 😉)