The way that you put "heads" on Momotaro Alter's chest does remind me of Goseiger headers.

My personal favourite potential future demon monsters in DonBrothers series:

- Kaizokuki (Pirate Demon) : Based on Gokaigers. A pirate-themed demon. He is capable to transforms into other demon monsters. I think the Kaizokuki is would be really cool as the new main villain, similar to Hiryu Kakogawa (Another Zi-O) from Kamen Rider Rider Zi-O series.

- Jyukenki (Beast-Fist Demon) : Based on Gekirangers. A kung fu-themed demon. He is capable to access various animal spirits to increase his kung fu fighting style abilities.

- Tokumeiki (Special Ops Demon) : Based on Go-Busters. A special ops-themed demon. He is capable of spying and fighting just like special ops members.

- Miraiki (Future Demon) : Based on Timerangers. A time travel-themed demon. He is capable to time travel to the past and to the future. Probably Miraiki are the final demon monster in DonBrothers series.

I wonder how similar Uchuki will be to a Zodiarts

What? The Kyurangers and Zodiarts get their powers from constellations.

Full size OP and ED came out a short time ago, and I have to say... I think Don't Boo! Donbrothers is proooobably, absolutely my favourite ED in sentai ever by a wide margin?! It's such a happy and upbeat, yet also cool and stylish song...

That specific part of the chorus that goes Wahaha has instantly materialised a giant grin on my face every single time.

Totally agree. It is an extremely good song!

I can't even believe how much I love this song

https://www.youtube.com/watch?v=1LGPVDhr67w

It's just this complete and total winner on every front for me. The way it's used in the show is great, the way the full version expands on it is great, the lyrics have both obvious thematic connection to the show and classic Sentai-y wordplay, which is all great, and before any of that, just as a tune, it's insanely catchy, which is, you guessed it, great!

https://www.youtube.com/watch?v=QV-e7XLiB_4

Don't get me wrong, the OP is great too, and I suspect the (probably deliberate) way the transitions between the sections of the song are a bit jarring will only grow on me with time, but that ED though.

And let's not forget that friggin Daito from Ready Player One is singing the songs. :lolol

What does Don Onitaijin mean in Japanese?

Well, "oni" probably means exactly what we think it does, and since it's a mech gattai, so does "jin", but the "tai" component still needs to be translated.