My hopes that Zi-O Trinity being revealed to belong to a fourth rider look almost completely dashed at this point, especially after settling Hiryu peacefully for now. Oh well.

I don't think both things necessarily clash against each others, especially if the Riders will have a more active presence now. Just look at how the Ryuki episodes double as the debut of Zi-O 2.

Or the Build episodes that also functioned as the opening to the season!

Bit of a rubbish example when those were not good episodes and gave me a very bad first impression of a season that I'm now currently enjoying a lot, but hey!

it's not really a problem of "returning actors take screen time from current protagonists".
It's more a problem of "By episode 40/45, Uhr/Ora and Swartz will still be in the "I create another watches/riders" mood instead of trying something else/them getting development".

We literally have 3 characters serving the same purpose, creating another watches.
While it was ok at first when there was room for character development and when we thought tribute episodes would take the first half of the show only, it's less acceptable now that we know it will be like that almost up to the end game, and that their personalities didn't really go forward.

In the latest scans, the next Rider arc is... Agito.
Also Another Agito is back as...Another Agito.

It's pretty clear at this point that Swartz is the one behind everything, even Oma Zi-O, and Hora and Uhr are just following him without knowing about his actual plan, and we have been getting Uhr development, which is how Woz became a Rider in the first place since he's questioning Swartz's plans now.

Besides, they have tried different things, it's just they always involve Watches, like creating the Another Riders from the future, where they couldn't just get Watches like they did previously, or creating Another Ryuga, a Rider based on a lost timeline.

For the upcoming episodes we seem to be getting more experiments, like creating Another Riders in the present and for the Agito arc we're getting multiple Another Agitos. White Woz also is now revealed to have been working with Swartz all along and apparently is doing something very dangerous to the entire world in the Blade arc.

I’m a little disappointed we aren’t getting Another Another Agito.

Ha! I knew those two fuckers were up to something. I fucking knew it!

After Another Blade and due to Another Agito being into the Another Rider category now, only Hibiki, Kabuto, Kiva & Drive remain.

But how to bring them out?

How about turning Kagami/Gatack into Another Kabuto. He always wanted to be before Tendou after all.

Chase's actor retired. How will Go/Mach revive him without that face!? Is there a chance for Proto-Zero to take on another human regardless?

Taiko Katono is freelance now. That's not the same as retired - and even if it was, Mutsuki's actor has come back post-retiring.