Can you help me understand the use case here of ProGet and Azure DevOps Packages? Basically I'm a bit hesitant to invest in continued work-arounds for bugs/quirks in ADO Packages.
The ADO Packages team has made it very clear they aren't interested in spec compliance (whether NuGet or otherwise), and that they won't support interoperability with any other tool aside from Azure DevOps. That makes sense because they want you to only and entirely use Azure DevOps.
We've already work around one quirk, and I have little confidence that fixing this quirk will actually solve the problem (i.e. enable the use case you're trying to build) and that it will stay fixed. In a few weeks/months, when they ship some new functionality, your hybrid-ADO Packages usecase may break again.
For example, when ADO-Packages decided to block "private upstream feeds" (I guess that's what it's called?), it disabled the use-case of having ADO-Packages pull from ProGet servers, or vendor's feeds, or anywhere other than NuGet.org or ADO-Packages feeds. Fortunately it's easy enough to work-around in ADO.
Bottom line, using ADO Packages this way may prove to be very fragile, and you may be better served by not using ADO Packages altogether. A lot of other users have made that decision, and ADO works fine (better IMO) without using their Packages feature... and it's highly likely it will continue to indefinitely.
Alex