Windows server 2016 vs 2019.

For support longevity, I'd go with 2019.  Some people still argue you should wait for the first service pack - which is just stupid advice since there are no more service packs.  2019 is like a service pack for 2016... which is like a service pack for 2012 R2... which was like a service pack for 2012.... which was like a service pack for 2008R2... which was like a service pack for 2008 SP2... which was a service pack for 2008 SP1... which was a service pack to 2008.

Basically, the point is, Microsoft doesn't re-invent the wheel with every version of Windows - they are constantly refining it.  New versions are just built on top of the same old code that is constantly patched and updated and occasionally replaced.  If you were to wait you'd just be waiting for waiting's sake in my opinion.

Much of the consideration deals with what the environment has in terms of applications and whether the vendors support the newer version.
You could/should buy a volume licensed server is which will entitle you to use the prior versions. You would be required to buy cal licenses whether it is 2016 or 2019, might as well buy the 2019 to carry your licenses into the future.

Since you have 2008R2, a dc should be using dfs-r versus ntfrs so transitioning should be completed before the introduction of the newer is ..

Point being, if you do not have any custom application that have sw requirements that don it include newer OS support, go for the newer versions provided the hardware you have supports them as well.

Btw, already have windows server 2016 client access licenses. Need to buy 2019 cal if l upgrade to 2019, is that right?

Yes, you would need 2019 Cals if you gave 2019'server, unless your Cals for 2016 included software assurance.

You can check if you bought volume license, you can check there to confirm whether your Cals updated to include 2019.

If you don't have SA but DO have 2016 CALs, then that is a potentially good reason to go to 2016 instead of 2019.  IF your network is small, then buying new CALs should be relatively cheap.  If it's large, it could amount to thousands of dollars.  Upgrading to 2016 and waiting to get, say, 2022 CALs (if that's the next version of windows server), might make sense before upgrading to the next version (or a newer version) of Windows server.