Bootcamp Assistant not seeing all available disk space

I can possibly get by on the 132GB partition that Bootcamp Assistant will let me make as is, but if reducing the APFS Container isn't super troublesome or risky, I'm willing to give it a try if it means I'll have more space to give to Bootcamp to have more wiggle room in Windows.

Between the two methods you mentioned, (restoring with a Time Machine backup or manually resizing the APFS Container) which one would you recommend? I have recent Time Machine backups and decent competency with using Disk Utility and Terminal, so I'm willing to try either.

That command fails with the following output:

Started APFS operation

Aligning shrink delta to 250,345,828,352 bytes and targeting a new physical store size of 749,999,996,928 bytes

Determined the minimum size for the targeted physical store of this APFS Container to be 848,459,440,128 bytes

Error: -69531: There is not enough free space in the APFS Container for this operation due to APFS limits or APFS tidemarks (perhaps caused by APFS Snapshot usage by Time Machine)

Should I maybe try removing the local Time Machine snapshots and see if that helps?

Snapshots for disk1s1 (10 found)

|

+-- Name: com.apple.TimeMachine.2021-04-20-124546

|   XID:  1533199

|

+-- Name: com.apple.TimeMachine.2021-04-23-121050

|   XID:  1547301

|

+-- Name: com.apple.TimeMachine.2021-04-23-133317

|   XID:  1547845

|

+-- Name: com.apple.TimeMachine.2021-04-23-143035

|   XID:  1548191

|   NOTE: This snapshot sets the minimal allowed size of APFS Container disk1

|

+-- Name: com.apple.TimeMachine.2021-04-23-153229

|   XID:  1548490

|

+-- Name: com.apple.TimeMachine.2021-04-23-185823

|   XID:  1549014

|

+-- Name: com.apple.TimeMachine.2021-04-23-223501

|   XID:  1549260

|

+-- Name: com.apple.TimeMachine.2021-04-23-233418

|   XID:  1549469

|

+-- Name: com.apple.TimeMachine.2021-04-24-100859

|   XID:  1549558

|

+-- Name: com.apple.TimeMachine.2021-04-24-101247

    XID:  1549596

I deleted those individual snapshots. If I was supposed to try the resizeContainer command again, it still fails with the same error as before. Currently the only snapshots are XID 1548191 and a snapshot that the computer just made a few minutes ago:

Snapshots for disk1s1 (2 found)

|

+-- Name: com.apple.TimeMachine.2021-04-23-143035

|   XID:  1548191

|   NOTE: This snapshot sets the minimal allowed size of APFS Container disk1

|

+-- Name: com.apple.TimeMachine.2021-04-24-105200

    XID:  1549749

And running diskutil apfs resizeContainer disk02s limits gives the exact same output as before.

Is there any particular reason I need to use Recovery over the internet instead of the built-in Recovery system? Is it because I'd be wiping out the whole internal disk before restoring from a backup? Will I need to reinstall High Sierra?

Because if I will need to download & reinstall macOS, I will have to wait until tonight to try since my internet is slow and it would likely take a while for it to download anything large from Internet Recovery.

Well, it looks like I'm stuck on step 2 here. I tried booting into Internet Recovery (holding Option-Command-R during startup) and it tried to load Internet Recovery for several minutes before failing with an error code -2102F. I'm guessing either my internet is just too slow and it gave up, or something else went wrong.

I can try hooking up the MacBook to my router via Ethernet cable and trying again at a convenient time, but I'm not sure how much it would help. My internet is old 3.0Mb/sec DSL from like 2003 (thanks to living in a mostly rural neighborhood) so that's the fastest it can possibly be. If that's simply too slow for Internet Recovery, it'll stall out no matter what. The only other option I have for internet at home is to hotspot my phone, which happens to have an unlimited data plan.

And yes, I do have more than one Mac. I also have a late 2015 iMac running Catalina and a mid-2012 Mac Pro also running High Sierra.

Good news - you can indeed use Disk Utility from within a bootable installer to wipe the hard drive. Good to know for the future if I ever want/need to do a clean OS install.

The bad news is that apparently the copy of Mojave on my installer was damaged and I couldn't proceed with installing Mojave after erasing the whole drive. Either the installer somehow got damaged while being put onto the USB stick, or the installer was bad to begin with (it had been sitting in my Applications folder for like 3 years, after all. I should have checked to make sure it was good or re-downloaded a fresh installer to be safe.) So for now I will just restore the drive from a Time Machine backup and go from there.

So it's finally finished restoring the Time Machine backup. On the upside, doing so resized the APFS container as mentioned earlier, down to a little over 400GB. This leaves me plenty of space to allocate to Windows. So the APFS container size problem is now resolved. I'd still like to update to Mojave before installing Windows, so I guess now the question is whether I should:

a) download the Mojave installer again and just update it from within the machine like normal, or

b) keep the plan of wiping the hard drive in order to do a clean install of Mojave and using Migration Assistant to bring in any specific bits of data that I want from a backup. In this case, I might need a guide on how to properly format the drive when erasing it to ensure that the APFS container doesn't get made any larger than what I want.