September is the date for the manufacturers to start being compliant although without the specifications being ironed out, I'm not sure how that's going to work.
Yes, the Mini 3 has an extended battery, soon to be released. But does just having the battery available for the mini 3, require it to use remote ID? Will probably require some legal interpretations of the rule for this, but it seems almost certain that if DJI offers the drone in a configuration 0.55 lbs or more, they will have to sell it with Remote ID starting mid September (unless delayed). If I don't use an extended battery , I assume remote ID won't be required. If your drone is flown for recreational purposes only and follows rules for not flying over people/moving vehicles and weights <0.55 lbs you will not be required to have remote ID in the US. Will the mini 3 know if an extended battery is inserted into the drone and then trip some kind failsafe that will either turn on remote ID, if it is allready installed in the Mini3 or will the operator need to purchase a remote ID device to fly with the extended battery? Only DJI knows what they'll do in this case. It seems like the drone should have the capability to understand what battery it has in it, so maybe they can force Remote ID on in that case, but we'll see what DJI does. They may take the "Safe" road and have it on all the time anyway, because they can't guaranteed payload size, but they do detect prop guards, so maybe they can leave it to the operator with small battery and no prop guards. We'll have to see. Almost certain you wouldn't need an external Remote ID device. DJI is planning for this for a while.
Last question: Are all new DJI drones having remote ID hardware being installed as of today's date? If so, will it suddenly activate in september? DJI claims that many of their drones including those that have been out for a while will be able to enable Remote ID via software update. As I noted previously, drones sold before 9/16/2022 have until 9/16/2023 to have Remote ID to be keep flying outside of special areas today, so DJI doesn't have to solve that for a bit. Who knows when they will activate it, and if they activate it will it be "optional" before 9/16/23 or not. Will be an interesting case for the Mini 3 because new Mini 3's will be required to have it on 9/16/23 but the older ones will not. Random guess, but would suspect DJI enables it for all of them so they aren't in a "manufactured date" issue with the FAA enforcement team.