SSS-Class Overlord: My Harem Rules the Realms (18+) - Chapter 55 - 3D Battle With Golem.

Yuna, a 15 years old girl started playing the world’s first VRMMO. She has earned billions of yen in stocks. She confines herself in her house playing the game without going to school. Today, a huge update has arrived. She obtains a non-transferable rare bear outfit. But the equipment is so embarrassing that she can’t wear it even in the game. Furthermore, when she answers the survey regarding the new update and relaunches the game, she appears in unknown forest in a bear outfit. Where is this? Email from God? Different world? If she wears the bear outfit she’s a cheat, if she takes it off she’s a normal girl. Her adventure starts just like that. Yuna began doing as she liked.

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.

The 7-year-old little crybaby met 11-year-old big brother who ran away from home.
[This man became the protector of redemption in his life]

The 16-year-old little dragon fell in love with a 20-year-old big brother.
[It’s a pity that the big brother didn’t recognize him as the little crybaby]

The 23-year-old single father and 27-year-old chief executive meets again.
[This time the little crybaby and his big brother are finally together]

Fortunately we met through hundreds of millions of people, Thank God, for letting us break through many obstacles and get back together again, falling in love with you and being with you again is the greatest fortune in my life.

- Description from Novelupdates