But I've checked with a Macbook from 2019 and it still only gives an MTU of 104 bytes. Although it should work from your list.Why exactly 104 bytes?

Depending on your peripheral it might be possible to get even 512 bytes if negotiated. This is possible starting from Iphone 7S. On mac OS this is different because the onboard bluetooth chip set does only support it when the macbook is after 2019. There are dongles like the Laird BT851. This can be used with older mac books. Although it does show support of 4.2 it only allows 27 bytes, since the dongle is not on apples internal DPLE list. This list is somehow hidden and I did not figure out how to put a dongle on this list nor the support was yet able to tell me what enables a dongle to get on the list.

Same here. It is even worse. I cannot even switch Bluetooth off. I've already contacted the support and yet there is no solution. Even a fresh installation via recovery did not work. Funny details: I have a M1 so I can run the nrf connect app from nordic and there I can scan. But If I try to scan with a Qt application or from the Bluetooth configuration it is not running anymore. Seems, that Apple did not test Bluetooth low energy at all in this release.

Updating the account information did not work out. What I've just noticed is that xcode is not able to download the symbols itself: It remains at 0 kB. Which should indicate that apple has an issue with providing the correct symbols. Seems to be the same story ever and ever again.

I've installed the Xcode 14.1 RC2. This now downloads the watchos 9.1 symbols. Now the apps can be deployed. Crazy, that this is not released together. Sorry but this were 4 hours of worktime.