Best practices for an in-place upgrade from Server 2008r2 to 2016/2019?
If you must stay with the same hardware/machines, then a two-step upgrade is your only option:
2008 to 2012, then 2012 to 2016.
But, if you have a virtual infrastructure or can use additional hardware, you can treat this as a migration rather than an upgrade.
Build new SQL servers and new IIS hosts, and then migrate the applications rather than upgrade the OSes.
I'm afraid not as there is no in-place upgrade path from 2008 to 2016. At least not from Microsoft. Not sure if there're 3rd party utilities out there to achieve this.