>>44147683H and Engage have completely different writers, 3H's main writing staff was KT and who work on musou games, so them writing about a war comes natural when these dudes basically live and breath Three Kingdoms/Sengoku the rest of the time. Engage is like Fates, just going over the credits there's fourteen writers credited in it.
Remember 3H was teased as FE 2018 way back during the FE Direct in January 2017 before it had to be delayed to June 2019. Engage was made staggered to it and was probably meant to target the 20th anniversary just like Awakening's (Japanese) release date was meant to coincide with FE1. Engage got leaked well before its 2023 release date to the point people thought it was fake(or made by Gust).
If I had to guess they probably planned for a rapid turnaround like Awakening into Fates and have Engage line up for the big 30th anniversary but weren't able to do it in time. Going off the datamine it's clear IntSys oversaw Silver Snow and Crimson Flower being made before leaving KT to their own devices which basically had them scrambling to recycle the existing maps into two more routes, which is why both AM and VW have vestigial route split data, and VW is just SS but has the contrived Nemesis fight at the end, where it would make more sense for the Black Eagles, the descendants of Saints, to be fighting Nemesis and the Elites. Compare that to 3 Hopes where they do have something of a route split and the GD route involves fighting the church.
As to Engage if I had to guess getting everybody on board with Unity probably hampered things a bit, vs everybody already familiar with the 3DS-era engine for Awakening and Fates. Either they were already straining to hit 2020 and then covid fucked things, or alternatively 2022 was the new target year to sync up with Awakening's 10th anniversary, given how much they openly talked about it being a giant boon and boom for the franchise in Engage's leadup to launch, but still couldn't make that date.
