If you're unwilling to undergo all the steps written down below, I also can provide pre-altered, basic saves as an alternative;
Your saves are located in '%LOCALAPPDATA%\TheAscent\Saved\SaveGames\'
BACK UP YOUR SAVES BEFORE EXTRACTING THE ARCHIVE AND OVERWRITING IT.
YOU HAVE BEEN WARNED.

WeaponsOnly.rar contains save that's at your very first visit to the Warrens, Cluster 13, with only the three exclusive weapons as an notable addition.
IntroAllWeaponsAndGears.rar has all the weapons and gears available in 210816 build and is saved at the very beginning of the game.
link: drive google com/file/d/1JnemHr3_E_lHojmxSqwiFdqKyyiVgo7g/view?usp=sharing
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Using this method requires 2 tools in your disposal;
1. Steam console: To download earlier build of the game
2. UUU (Universal Unreal Engine Unlocker): To force enable ingame developer console.

Steam console is already packed with your Steam client and UUU can be downloaded from an official source, a simple one-time Google Search would suffice to get that link.

While I didn't try, if you're already in the location that only exists in after release update build(Update 2+) your game will likely crash due to your save being incompatible. Before doing anything, do save in locations that's present in the base game. Some of your items will also disappear for the same reason. Think of it as a small price to pay.

To use Steam console, press Win+R and input steam://open/console.
Then type or copypaste the following command in console.
download_depot 979690 979691 5792437286988117460
(Thanks to CrimsonDemon for the correction!)

After your download is finished, rename your former installation folder(to re-use it later) and replace it with freshly downloaded older version of the game.

Still stuck?
For more comprehensive steps in downgrading your game: check the below guide.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=889624474

Boot your game and load your save. After that run UUU and select your The Ascent executable process, then Inject.
Now you can bring up ingame console by pressing the tilde(~) key.
Type or copypaste two following commands:

EnableCheats 1 (press enter after)

NG_GiveAllItems 0 1 (press enter after)

Check your loadout to see if all weapons are added correctly. Note that the PS4-exclusive RPEG-33 is named as En Bomb in this older build.

Delete your older-build installation folder and rename your up-to-date build folder to its former name so it can be recognized by Steam again.

For all I know this is a permanent alteration to your save and you do not have to repeat this process again for that save. Enjoy.

Thanks to wmxp on FR and Reddit for publicly explaining his own findings, it would have been impossible for me to publish this guide without his years old comments.

Q: Does this method disable achievements?

A : No.

Q: Am I going to be ba-

A: No you're not. Game doesn't even have an anticheat solution(as it should be).

Q: Any unwanted side effects?

A: Can't tell, but if you encounter something please do notify me so I can specify it in this guide.