I Have a Mansion in the Post-apocalyptic World - Chapter 1556 - The Ambition of Loki Jr.

A scientific genius named Thomas Harrier was transmigrated to an imperial prince body after a car accident that took his life. He found out that he is in a world similar to our own with a timeline akin to early twentieth century. The body that he is occupying was the imperial prince of a failing empire who suddenly rose to power after a successful assassination attempt of his father. Unaware that the attack is just the beginning of what is to come. He faces many challenges throughout. Will Thomas Harrier be able to live his second life in peace while running an empire and getting it back to its former glory? Or will he succumb into a conspiracy lurking within the country's borders?

Expect some:

Cold war era techs.

Super battleship with advanced electronics, propulsion, and fire control systems?
Helicopters?
Missiles?
Modern medicines and equipment?
AC-130?
A nuke?
Challenger or Abrams or Leopard?
F-4 Phantom or MIG-21?
Colored TV's.

Everything!!!

PS. Cover not mine, I just found it on google.

Adrien’s tale is a thunderous symphony of vengeance, mystery, and ascension. Awakened with no memory but wielding a shadow-forged blade, he traverses a fractured world where gods rule through fear and silence. His forgotten blood burns with the legacy of the Godslayers—beings once feared even by the divine. As he hunts corrupted gods and unearths shards of unimaginable power, Adrien evolves from a wandering enigma to a force capable of defying fate itself. Twisting through betrayal, hidden empires, and awakening legends, his journey is not just one of survival, but of reclaiming divinity to break the sky itself.

In the age of the Warring States, the world is at war!

At this moment, a great empire is about to be established, and at this moment, this empire is at stake.

In this vast world, Young Master Qin shouted loudly, “I want this great Qin Wanshi–!”

- Description from novelbuddy

An incredibly athletic older brother, Tenryu Taiga. A programming genius girl, Tenryu Fuyuki. The two siblings separated for eight years ended up living under one roof again after facing the problem – a lack of funds for living expenses.

Fuyuki proposes a way to earn money, a core system deeply involved in people’s daily lives, a game that awards you money as a prize for defeating malicious bugs in cyber world, the 《Aries》structure. Taiga after becoming a warrior in the game relying on his inborn senses, is puzzled by the unfamiliar sensation.

One day Saionji Rui, Fuyuki’s best friend appears and is attacked by a mysterious bug― “I’ll be waiting, my prince.”. Hybrid fantasy battle of cheating siblings begins here!