There should be little current through the battery while the phone is plugged in, virtually none if the charger can meet its peak power draw. If degradation over time is a concern (perhaps it boot loops if the battery is very dead), a charge-limiting app set to 50%, the level recommended for storage and used in the Microsoft Surface UEFI Kiosk mode, should maximize life.

If the phone is rooted, you can use Advanced Charging Controller to cycle it between min/max values. That would only prolong it, of course.

Usually these don't like booting without the battery, but maybe there's a way to provide data-only USB connections and a lithium-level voltage internally. Often the early boot stages will interrogate the battery level and refuse to boot if it's too low, but it's very much model dependent.

had two batteries "explode" (ballooned ¹) because of this, then I switched to another phone and knock on wood the battery has not yet exploded (about 2 years).

¹ This is probably why you should not cut off the battery plastic sealing like some DIY guides tell you to do, it's probably there for a reason.

You can leave a phone plugged in all the time with no issues, the same way you can do the same with a laptop or any number of the other ubiquitous USB-chargeable things these days. The only time it's an issue is if the device was _extremely_ poorly designed.

Some good phones will even notice that they are plugged in all the time and drop the charge level to 80%, where most batteries are happiest to stay for long periods of time.