As a contributor I would like to say using 5.18 is fine as long as there are still active backports, but **using Qt 5.12** is problematic and many programs has requirement on 5.15 nowadays. To my knowledge openSUSE Leap 15.3 is still using Qt 5.12 to keep compatibility with Plasma 5.18, as a result, some latest programs cannot be packed for Leap 15.3 anymore.