The preferable options are that one service per hour off the Beenleigh line operate via Tennyson into and out of the CBD,
I don't like this idea particularly. If you mean 1/hour Beenleigh-city via Tennyson and 1/hour Beenleigh via South Bank, what would service Yeerongpilly-Dutton park, or are they only getting an hourly frequency?
Thinking some more about this, there's nowhere I am aware of between Manly and Park Rd to turn trains around besides Manly, so a Manly-Ipswich line service would involve a lot of low patronage running. Also, that would not pick up the Coasties unless they returned to stopping at Yeerongpilly off peak, and it's not too bad to change at Roma St for such commutes. I actually think that a Sunnybank-Richlands service would make some sense, but the lack of wires on the 4th track would be a big problem to do this in peak. If Richlands receives only 2tph off peak to the city, that could be supplemented by 2tph to Sunnybank with a change at Darra for the city. It also picks up Coopers plains which makes sense for interchange with the Coasties.
Of course, I am assuming that a Sunnybank turnback exists! Altandi would be even better.