You only really need this mod to play True Solo. It allows you to disable bots and optionally to customize game difficulty.
You can customize everything in mod options(F4), or quickly choose from 2 presets when you open the map screen:



True Solo Preset

This preset disables the bots, patrols, and doubles damage you do to bosses and lords(ingame name for the map finale bosses).
Also it restarts the map on defeat, so you don't have to go back to the inn.
This is not usually seen as making the game easier by the True Solo community, since patrols and bosses are usually more of a time investment than truly difficult. A lot of weapons struggle with boss damage, and the way dynamic difficulty system works in Vermintide, a boss that has been alive for some time will stop new hordes from spawning.
Only people who find using this setting contentious have never actually played any True Solo.

No Bots Preset

If you want pure vanilla experience just choose this preset.
Also consider enabling the Restart Map preset that is below it.

Customization

If you're only starting with True Solo, trying to win can be incredibly frustrating. After all, it is a game intended for 4 players.
You can use Spawn Tweaks to tweak a lot of difficulty settings. Notably for True Solo the disablers(Assassin, Packmaster, Leech) are the biggest threat, so tweaking them is advised. Either disable them completely using Disable Specials By Type, or use the Assassins Give Up, Leeches Always Fail, and Packmasters Always Fail to still have them in the game, but so you don't immediately lose when you get disabled.
You can also disable bosses, hordes, or everything but hordes, and so on, if you want to focus on practicing a certain game element.
Also, not Spawn Tweaks related, but try a difficulty below the difficulty you're normally comfortable with.
True Solo is a way to challenge yourself, and have fun with the game. There is no wrong way to go about this.

Spawn Tweaks


This mod makes it so the level automatically restarts when you lose, skipping the return to the inn. Very much recommended to save some sanity from loading screens, since True Solo involves a lot of losing. This option is now also inside Spawn Tweaks.
Most of the options inside the mod options are not needed nowadays, since this was the original mod for True Solo.
It has the same option to kill bots as Spawn Tweaks with Auto Kill Bots, and notably the option to get a warning when Assassins or Packmasters spawn on the map, either via text(Assassin/Pack Text Warning), or via a audio que form the hero you're playing (Assassin/Pack Hero Warning). This is a workaround for an issue of them silently spawning, which can happen.

True Solo QoL Tweaks


Not needed nowadays to True Solo. This was the original way to kill bots.
Killbots mod adds a chat command /killbots, and a hotkey you can choose in mod options to manually kill bots.
If you have True Solo QoL Tweaks the UI elements of dead bots will disappear as well.

Kill Bots


Not needed if using True Solo QoL Tweaks, since it does this automatically.
Adds a chat command /restart, and a hotkey you can choose inside mod options to restart the level manually.

Restart Level Command




If you don't have perfect weapons and trinkets, use this mod to create them on the Modded Realm. They'll have 300 power and max properties.
Save Weapon will save the weapons you've created between game sessions.

Give Weapon
Save Weapon


Allows you to pause the game, either via the chat command /pause or via or hotkey in the mod options.

Pause


Allows you to spawn enemies for practice. Spawn Packmasters and Assassins and practice dodging them, or bosses.
Has a hotkey that allows you kill enemies around you as well, very useful.

Creature Spawner