If you really want to watch all the series Decade pays tribute to, watch only Amazon -> Black -> Black RXFinished Kamen Rider Build over the weekend. Easy 10/10. One of my new favorites, just an absolutely excellent show all around. It started a tad slow but once it got moving, it never stopped. Basically cried through the last like, 4 episodes. Everything was executed so well and the emotional beats hit really hard. This one is definitely gonna stick with me fore a while, Now that I've seen W through Build, I jumped back to Kuuga and will now work my way through Phase 1! Only 5 episodes in but really enjoying it so far.
Debating stopping at Kiva and watching Showa before Decade seeing as there are a few Showa era Riders that appear in Decade.
...but then Amazon has a recurring character, and RX has a crossover with previous Riders. Oh no, lol.
Still, I'd watch only those three. Even Amazon I'd say it's not really necessary, since Decade's Amazon has little to do with the original.