[Aoi Kurenai]

More often than not, tokusatsu writers have an idea of where their show is going and have most of the major story beats in their head. Zyuranger, for example, either has the most or second most two parters in all of Sentai with most of them contributing to a larger narrative. Dairanger is focused on various characters having their own arcs, Kakuranger is split into two distinctly different halves. (Kakuranger in particular is more subtle but there's a narrative that follows the team seeing the enemies as enemies and then realizing that wasn't necessarily the way things had to be too late for it to matter) Double's planning stages were meticulous as heck, Inoue had most of Agito worked out early on and decided to write all but a single episode because of that. Noburo Sugimura eventually left tokusatsu because, after Ohranger, the emerging market of story heavy video games offered him the creative freedom earlier tokusatsu gave him.

Shows won't always be planned out to the extent of say, Gaim, but most tokusatsu writers do go into these shows with something of a master plan. They're very malleable, yeah, this is why you get quirks like Natsuki's precognition never showing up again after the first episode. (Sho Aikawa, Boukenger's head writer, felt it made her too powerful of a character) Or why certain characters who turn out to be more popular than expected stick around longer than intended (Date in OOO) or get new forms. (Kaito in Gaim)

There are even writers meetings so the people who only contribute a small handful of episodes have a road map to stick with. This one is a bit more recent for certain franchises, Ultraman didn't start it until Gaia but that's because Ultraman's very nature was episodic.

Though it also depends on how involved your producer is. Naomi Takebe (OOO, Gaim, Go-Busters, Ninninger) often likes to stick to the day to day aspects and gives a writer greater control. The one exception I can recall is the Kogami stuff in OOO. Sho Aikawa didn't like where Decade's producer wanted it to go and ultimately stepped down as head writer after 14 episodes due to this difference in vision.

Shinichiro Shirakura, the architect of the Heisei formula, has a relationship with Toshiki Inoue that gave the writer a great deal of control over the story. Compare this with Den-O where after years of falling ratings and toy sales, Shirakura wanted it to be about the characters rather than the story, so Yasuko Kobayashi adhered to those wishes. As much as people dislike Kyoryuger, its producer and writer got along famously and hashed out a very detailed vision for it early during production.

In the 90s, Toei prided itself on its writing staff's credentials and even made it a point to publically declare "Toei is not a place for new writers to get started" after they began to gain such a reputation. It's even theorized that Yasuko Kobayashi wrote most of Juukou B-Fighter using her mentor's name because of this.

All that said, I think it's possible to have a detailed idea of your show, the story, and its characters, and still push out an episodic show due to Sentai running for a full year. This is something shows, even kids shows, rarely do these days.

This turned out to be a lot longer than my initial "Yeah, they tend to plan stuff out" post, but I sorta just vomited what I know all over the place so you'll have to excuse my incoherent ramblings. (also it's hella late for me and I just got done with a rigorous study session so I'm fried, but I hope yall got the gist of what I was trying to say)