As there seems to be no progress at all by Plesk for the bugs EXTSSLIT-1879 and PPS-15436 we decided to implement our own work-around.
To share wildcard certificates among subdomains is buggy with heavy impact on our client's satisfaction.
To avoid this shameful bug of a so called "server management platform built with you in mind." the only way is to take the renewal process in your own hands. Just scan your sites and certificates with an appropriate script and renew the wildcard certificates 5 days before Plesk would do it on its own.
Not only our clients are speechless, when we have to tell them: "Sorry, but our server management platform is not able to renew your website certificates appropriately. The bug is know since 6 months, but Plesk has no technicians or time or mood to take care this issue."