What are some excellent free games/total conversions that are worth playing the whole thing?

Beyond All Reason is an open source RTS similar to Total War. Here is their website.

If you haven't already played it, DOOM: The Golden Souls is probably the best total conversion you can find. Just S tier all around. Adventures of Square is also pretty good despite looking a bit simple.

If you're looking for free games in general, might I recommend Megaman Maker? There's an absolutely insane amount of content, and the level editor itself is very fun to use.

Enderal, a total conversion mod for Skyrim. Free, just requires ownership of the skyrim base game or special edition (there's a version for either).

Has a whole new map, reworked classes, changes to combat, banger OST (here's my favorite, hits incredibly hard when you're listening to it in-game at a tavern) and the story is just incredible. I still think about it, and the choices I made, years later. Dozens of hours long, moreso if you are a completionist.

Mount and Blade: Warband has multiple incredible total conversions. I've dumped a lot of time into Prophecy of Pendor and The Last Days, probably more than the base game.

For actually free games there are so many options that it really comes down to taste. Unciv is a fantastic reimplementation of Civ 5. Super Auto Pets is a fun casual auto battler. HoloCure is a really good Vampire Survivors-style game themed after Hololive vtubers. There are tons of MMOs and shooters that are F2P and good, but I know most of those from hearsay rather than experience.

It's been a long time but one of my favorites was the Star Control 2 remake Ur-Quan Masters. I believe there is a free HD version out there, too. Plus if you like it, the OG creators have current projects you can support (they left that IP after 2).

Not sure what genre it is; certainly not action packed like your examples but those space battles were fun.

I really enjoy Mindustry, it's kind of like Factorio

https://mindustrygame.github.io/

I really enjoyed The Last Days of the Third Age, a lord of the rings TC for Mount and Blade: Warband.

It takes Warband’s medieval lord sim and focuses hard on the warfare side of things with lots of huge battles on giant maps, but even tiny skirmishes can affect a faction’s strength. (And with so many factions on both sides of the War of the Ring, you need to pick what to focus on)

I recently found out about the Zelda: Link to the Past randomizer community. Essentially there is a method to modify the original game to randomize the item locations and other aspects of the game while ensuring the game can be completed. Each game has a seed so you can look it up if you get to stuck. I played through it once and it was a really interesting challenge since I didn't get the bow until 60% of the way through the game.

The Elder Kings 2 mod for Crusader Kings 3 is pretty amazing. It lets you roleplay in the Elder Scrolls universe, but instead of a Skyrim-style rpg, it's a grand strategy game with intricate political and relationship mechanics.

There are several good total conversions for Sins of a Solar Empire: Rebellion. Star Trek Armada 3 is the most complete overhaul I’ve played for it. I’m a big fan of Halo spaceships and the Sins of the Prophets mod is the closest thing I’ve seen capture the asynchronous battle between UNSC and Covenant ships. There are others for Star Wars and Stargate that I hear are good, but I have not tried them.

Sunk 2000hrs into Warframe without paying a dime. Also Path of Exile is a nice fair free 2 play game. For total conversions try Fallout new Vegas and mount and blade