Yeah it's obviously true that 40K has a huge amount of lore and it's not possible to easily replace that. What I'd suggest is simply having a group of lore nerds sit down with the 40K lore and agree to an approach of either time-skipping forward (aka 45K) or freezing the timeline at a certain point (pre-Primaris marines for example) and agree to that.

Then you work on developing basically a set of interchangeable terms e.g.

Space Marine = Space Marine (Not copyrightable)
Imperium of Man = The Galactic Dominion
Chaos = Ruin
Demons = Demons
Warp = Nethereum
Eldar = Sylvani
Tyranids = Raveners

Start with the big concepts and work your way down. Stuff would feel different even if you did a perfect 1:1 copy to start with, and that's assuming no one wants to add or subtract something.

I ran this through ChatGPT just to present how this could look:

The Galactic Dominion (Imperium of Man) The Galactic Dominion is a vast, galaxy-spanning empire of humans led by a powerful, god-like figure known as the Celestial Sovereign. The Dominion is a highly militaristic and religious society, with a strong emphasis on loyalty, obedience, and the eradication of all threats to humanity.

The Eternal Forgemasters (Adeptus Mechanicus) The Eternal Forgemasters are a faction of tech-priests who worship ancient technology and machines. They are responsible for maintaining and producing the advanced weaponry and equipment used by the Galactic Dominion's military forces.

The Astral Warriors (Space Marines) The Astral Warriors are genetically engineered super-soldiers who serve as the elite fighting force of the Galactic Dominion. They are organized into chapters, each with its own unique culture, traditions, and specialties.

The Devouring Swarm (Tyranids) The Devouring Swarm is an extragalactic race of insectoid aliens that consume all organic matter in their path. They are driven by an insatiable hunger and are capable of rapidly evolving and adapting to new threats.

The Ethereal Crafters (Aeldari/Eldar) The Ethereal Crafters are an ancient, psychically-gifted race of aliens who once ruled a vast empire. They are now divided into distinct sub-factions, each with its own agenda and goals. The Crafters are known for their mastery of advanced technology, psychic abilities, and their ability to manipulate the threads of fate.

The Necro Continuum (Necrons) The Necro Continuum is an ancient race of mechanical beings who once ruled the galaxy. After a long period of dormancy, they have awakened to reclaim their lost empire. The Necro Continuum possesses advanced technology and the ability to self-repair, making them a formidable force.

The Verdant Tide (Orks) The Verdant Tide is a race of green-skinned, warlike aliens who live for the thrill of combat. They are known for their brutal tactics, ramshackle technology, and their ability to grow stronger and larger as they fight.

The Immaterial Realm (Chaos) The Immaterial Realm is a parallel dimension of raw psychic energy and chaotic entities. It is home to the Chaos Gods, powerful beings who seek to corrupt and control the mortal realm. The Immaterial Realm's influence seeps into the material world, twisting the minds and bodies of those it touches.

The Ethereal Collective (Tau) The Ethereal Collective is a young, technologically-advanced race that seeks to unite the galaxy under its banner of the "Greater Good." The Collective is known for its advanced plasma weaponry, sleek armor, and its use of diplomacy and coercion to expand its influence.

The Dread Kindred (Dark Eldar/Drukhari) The Dread Kindred are a sadistic offshoot of the Ethereal Crafters who revel in causing pain and suffering. They are masters of fast-paced, hit-and-run tactics and use advanced weaponry and technology to terrorize their enemies.

The Unyielding Vigil (Sisters of Battle/Adepta Sororitas) The Unyielding Vigil is an all-female military order dedicated to the worship of the Celestial Sovereign and the protection of the Galactic Dominion. They are known for their unwavering faith, their powerful armor, and their use of flame-based weaponry.

Not great for 1 try, but some aren't bad (I like "Dread Kindred" for Dark Eldar as it has some vampire vibes).