You say you prefer z/OS over mid-range, so why not go the z/OS route? You MUST enjoy your work or live an unhappy life.
I know many people who have certs in database, and specifically DB2.
But I do not possess any certs. I’ve been working with DB2 on z/OS (and its predecessors os/390 & MVS) since 1987 (that would be version 1.2).
Some people like to hire folks with certs. Some do not care. I am one who does not care. I ask detailed, technical questions during interviews. Some people with certs cannot answer them.
The only time certs would matter to me is that it shows a willingness to follow up on your own. It shows some initiative. But it does not truly prove how good one is. It proves book learning. it proves some knowledge. But it does not prove the ability to think outside the box when you get a 3:00 am phone call.
Your 5 year old knowledge is still good. All you have missed out on is the “new” stuff. But the old stuff (99%) is still valid. VSAM is still VSAM, and so on.
As far as the demand for DBA skills, well that varies a LOT depending on location and money and … Right now, there a lot of unemployed technical people. But there are also want ads looking for people. Would you move? That will certainly help with finding the demand.
Much of DB2 knowledge is transferable between big DB2 and little DB2. I haven’t worked much with little DB2 since the OS/2 days back in the 90s. But a lot of my knowledge is still valid – but I am out of practice, of course. But things like the catalog are almost the exact same. concepts are the same. hot & cold backups are the same (although the files are different).
Other things are not interchangeable between the 2 – buffer pools, EDM pool. And command syntax (not SQL, but COMMANDS) differs a LOT. But they accomplish much the same stuff.
I say follow your heart and go with Big Iron DB2 but also learn some little DB2.