I've finally fixed it! I found a bunch of errors related to "AutoUnlock" in the Console that was hinting to me that there was some invalid state on my Mac with keys and plists not being reset properly. After clearing/resetting them, I can now successfully pair my watchOS 7 Series 3 to macOS 10.15.6.

Steps (follow at your own discretion)
Open "Keychain Access"
In "View", enable "Show Invisible Items"
1600317739824.png
Search for "Auto Unlock"
You should see a whole bunch of application passwords for "Auto Unlock: XXXX's ..."
1600316509935.png
Select all records and delete (this will reset/disable auto unlock on other Macs if you use multiple Macs)
Whilst still in "Keychain Access", search for "AutoUnlock" (no space)
There should be 4 entries for "tlk" "tlk-nonsync" "classA" "classC"
Screen Shot 2020-09-17 at 1.26.18 pm.png
Select 4 records and delete (don't worry if they re-appear, the system repairs this automatically)
Open "Finder" and navigate to "~/Library/Sharing/AutoUnlock"
There should be two files "ltk.plist" and "pairing-records.plist"
1600313712532.png
Delete both files
Open "System Preferences" and try enabling auto unlock. You may need to enable it twice, the first attempt will fail.

我自己做了个重置图标的自动化，但还是想一劳永逸地解决问题。

watch 解锁 Mac 的问题，给你个解决方案
https://forums.macrumors.com/threads/watchos-7-beta-5-unlock-mac-doesnt-work.2250819/page-2

Safari 重度用户求一个。

昨天和 Apple 的高级技术专家通话了也没找到解决办法，今天运气真好！感谢 1259 ！

岳父教女婿做人？不是自家孩子，哪些所谓的长辈们还是少开腔好，个人有个人的行事风格，说这说那只是显得自己有存在感而已。

锤子哟，刚刚联系 Apple 官方客服，告诉我最新版本号就是 2B588 或 2C54，说没有更新的固件。

新 Air 每天常驻软件：Safari 、Office 、reeder 、 钉钉 、微信 、备忘录 、提醒事项 、邮件 、onedrive 、istat 、surge 以及其他等等，还没遇到过风扇狂响。

别在意，刚入的 2020 MacBook Air 电池循环 16 次，istat 电池健康度一会儿显示 99% ，一会儿显示 98%，已经不关注了。