So, in regard to my own question, here is what I found works:
The solution:
I looked at the USB socket and cable and first cleaned them with isopropyl alcohol and then shifted and pressed on the connection in different directions and with different force. It can clearly be seen that the current consumption increased up to 1000 mA (the nominal current draw I would expect from an iPhone with <50% SOC), but was not stable, when releasing the pressore or grip on the USB connector.
So I tried different positions of the connector in the socket and various positions of the cable tied to the MacBook case and in some instances the current draw stayed constantly high (~800-900 mA). Interestingly the freezing issues subsided when the current reached this high and stayed off even, when the current draw would subside back to 400 mA if the pressure on the connector was released. In this case the current would falls all the way down to 260 mA not charging the iPhone. Only physically removing the cable from the socket and retrying the fit would resume charging with showing the freezing issue until the charging was consistently above 800 mA.
So at last I found a position on the right USB socket that utilized the full charging power with 950 mA. The freezing stopped and the iPhone charged normal (and fast). The tinkering on this port might have lead to solving the problem for now as I can repeatably use this port now to charge the iPhone and not have to stop the usbd process. The other socket still shows the same old problem.
The most likely reason for this issue:
So from this investigation and experiments I can confidently say that the issue with reconnecting iPhones on my MacBook Pro from 2015 is due to high contact resistance on the connection prongs inside the USB socket and USB connector from the cable used. This also explains why a new lightning cable or even third party cables might resolve this issue for a certain amount of time.
A permanent solution is probably rebending the socket prongs and cable prongs inside the connector/socket. I believe that they are manufactured with a certain preload which subsides with constant use very quickly. Also maybe the connectors are gold plated to reduce contact resistance and repeated use scrapes off the plating from the connector (which can be seen on cheap USB sticks). In any circumstance the connection surface area of these prongs is very very small and might lead to high resistances (as USB was never intended to supply more than 500 mA). The iPhone charging chip is obviously sensitive enough to react to the reduced voltage resulting from the contact resistance and disconnects the iPhone as a precaution.
As I will replace my battery in the next weeks I will open the MacBook and work on the USB sockets. Don't do this while the battery is connected inside the MacBook as you might short something.
Maybe someone else can try to replicate my results and report here, that would be awesome. This problem bugged me for years and nearly prompted me to by a new MacBook Pro (until I saw the current prices - I nearly fainted). Good to have a real solution.
And many thanks to the author of my referenced thread here, disabling the usbd process kept me going for years on!!!
TL; DR
A solution might be to clean or bend the connectors contacts within the USB socket. For me cleaning solved the issue on one port repeatably! A deeper cleaning or reworking the sockets might fix it permanently.
