How to use a MacBook always wired like a desktop/MacMini?
Sell it for parts and buy a new or used Mac mini. Or do some research and testing so see if you can live with throttling after you suss out today’s upgrade costs.
The system has to detect a healthy battery since the power supply can not provide enough power to run without throttling. Apple doesn’t do this to be mean spirited, the battery is needed for electrical reasons. You would need a different power supply and to reprogram the controller chips. A cool project, but it’s cheaper to sell the laptop for parts to someone that wants to put a battery in it. That’s a great Mac for the price of a battery. Also, as parts age or you disable the safety and management code on temperature, the risk of a melt down or premature failure goes up. Removing the battery makes a possible fire much less risky so that’s the only plus to having no battery in a portable. It might even restore stability to your setup.
Same as the drama about iPhone throttling. All CPU can over draw their battery or power supply or overheat if the laws of physics are not obeyed.
Throttling is there to maintain required voltages and temperatures, not some marketing stunt or planned obsolescence nonsense. You can measure this in a lab if you don’t trust “the man”. (And no one should blindly trust “the man”, be they Apple or Dell or Intel or some young engineer making their first chipset / home repair.)
Also, don’t overlook alternate OS. We’re starting to load unix or even “Windows” on some old Mac where we can’t run Catalina. Reuse before resell before recycle if you can.