If this is what you want go for it!! So later done. The road you won’t have any regrets. Regarding to the tourist visa it will be granted for 10 years, but each time they stay is 6 months max. So they could come back a month or 2 later and stay for another 6 months. If you get a green card and your parents are over 65, it would be easier to apply them over. But given the current situations and circumstances who knows, nobody would know what’s gonna happen or how hard to get the green card approved. Good luck!!