F5 3600 OS Upgrade from 11.2 to 11.6

we can't give you a recommended version. it depends on your needs and internal policies. personally i play it safe and use 11.5.1 currently if there isn't a huge need for any of the features of 11.6.

and yes you can upgrade from 11.2.1 to 11.5.1 or 11.6.0 directly.

as for your upgrade, if you have a partner you could ask them for advise and assistance. they probably done an upgrade before and know a little bit better what can turn up. but even then it remains not fully predictable.