For the hosts, you already have an answer. For the 6000 Checks: Most likely Checkmk’s discovery will find 80-90% of what you need out of the box. Maybe even more. I did the same thing you are trying to achieve (Icinga 1 in my case) with double the scale and it is doable. It sounds daunting but really is not that bad. And down the road there will be all kinds of things that will make you love Checkmk compared Nagios-like solutions.