Wont charge or turn on

A USB ammeter would be really helpful in this situation so you could see the voltage and current draw the phone is receiving to know if it is truly charging properly or not. Without some additional diagnostic information, it’s hard to say what the problem would be so we can only really guess the likely suspects based on the description of the phone’s behavior.

Since you’re already tried multiple cables, wall adapters, and even wireless charging, we can at least rule out faulty charging accessories (assuming the ones you used were known-good and working) and possibly the micro-USB charging port.

As you suspected, you could start with a battery. The Samsung Galaxy S6 is old enough now that you very well may need a new one. Should the battery replacement not solve the problem, you would be looking at a potential board-level problem involving the charging circuit which would require additional diagnostic tools and equipment to further troubleshoot.