I am a BMW technician. I have changed fluid on THOUSANDS of BMW's of all ages and types. Including yours.

When you bleed, or more accurately, change the fluid in your 2018 GS, you will be changing the fluid in the ABS pump if you like it or not. It's an integrated system.

You should change it like any conventional braking system. There are no separate bleed valves on the ABS pump like in the older pre-2006 ABS pumps.

A 500ml should be enough to change the fluid in the entire system. When doing this job in a dealership, you are given one 500ml bottle for the entire task.

I would drain the reservoirs (by pumping or syringe) until they are almost empty and then clean the reservoirs if there is any gunk in them. Then fully fill them with fresh brake fluid. On the front , you gently squeeze the lever 1/12 of its travel to remove any trapped air in the master cylinder and squeeze the hose to the rear reservoir to do the same. Then I would pump this new fluid until it is almost empty again. Then you fill to the max limit on your reservoir and replace the cap.

Like you, I keep going until the bottle is empty. Why wouldn't you. With one 500ml you should be able to flush about three rear reservoirs and two full reservoirs on the front. One per calliper.

As you say, an open bottle shouldn't be used after six months or so. BMW advise brake fluid be changed every two years or 12,000 miles IIRC

If you read BMW's instructions in their workshop manuals (which you wont have or have access to) you should have your pistons pushed fully into the callipers before you do this. And you would also perform a bleed test on the older pumps.

However, NO BMW dealer does this that I know of. It's totally un-necessary.

When I have changed ABS pumps or entire brake line systems (so starting with a dry bike), it will generally take one 500ml bottle to fill the entire system including some bleeding. And I will use another bottle to bleed again just to be sure.

You don't need any fancy one-way bleeders, Mityvacs or other vaccuum bleeders for this task. Just some clear hose and an old bottle. It's actually easier like this.

The MOST IMPORTANT thing is to not introduce air into your system when changing fluid.

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Did some trips.
Rode some bikes.
Fix them for a living.
Can't say anymore.