There are several Church of Ireland Churches dedicated to St John in Belfast:-
St John's, Whitehouse/Shore Road (opened 1841)
St John's, Malone (1842)
St John the Evangelist, Laganbank (1853 - moved to Orangefield/Castlereagh Rd in 1937)
St John the Baptist, Stewartstown Road (1859)
As you see the Laganbank church has just moved location and all are still in operation.
Malone and Orangefield/Laganbank marriage records for 1922 are still held in the churches, St John the Baptist records for that period are in PRONI (but inaccessible during their move) but Whitehouse records are on microfilm at Cregagh library and looks like the only one you haven't tried yet.