If your device is rooted, this Magisk module claims to do what you are asking (not tested by me)
How can I force "Battery Saver" mode to remain enabled, even when charging my Android device?
Apparently, this is not possible.. You are trying to make changes at a system level by creating an option that doesn't exist (always use battery saver mode), whereas other answers that you quoted in comments were only invoking existing options under some conditions). Hence, what you are left with is the option of making battery saver more effective by clubbing it with other options like doze etc.
Quoting from XDA thread Meet Buoy, a FOSS battery saver configuration tool!
Sticky Low Power
Buoy also enables something called "sticky" low power mode. Usually, when the device is plugged in and unplugged, the low power mode is then disabled. However, sticky mode re-applies low power mode afterwards to continue saving battery.
(Emphasis supplied to highlight why it is not possible)
Therefore, it is better to customize the battery saver state to maximize on battery availability while reducing charging duration
- Buoy is open sourced with a bunch of features that make battery saver mode behave from typical to aggressive, thereby increasing your battery life and reducing your charging time.
This appears to support only upto Android 12 but a developer has forked it by building upon it and naming it Saver Tuner (download from GitHub Releases/F-Droid)
Screenshots show the amount of customization possible