I would cut Southern services to 1tph to free up paths, and make the remaining Southern serve both Hove (for Brighton) and Gatwick Airport, as it did in the 90s when first introduced. To avoid increasing journey times to Gatwick, I'd also make it limited stop, e.g. after Havant call only at Chichester, Barnham, Worthing, Shoreham, Hove, and Haywards Heath - again much like the 90s. I realise this would also require some Coastway tweaking.
I'd keep Portsmouth-Cardiff, that is a very long standing service and it wouldn't be good to remove it now.
Much like SWR's original plans, I would add a second semi-fast service an hour, timed to leave Southampton around xx20 which would give reasonably good spacing with both the fast and the stopper. This could call at St Denys, Woolston, Netley, Swanwick, Fareham, Cosham, Fratton, PSS, PH.
This way you get 2 fastish tph from Southampton-Portsmouth, and also 2tph at the larger intermediate stations.
I'd keep Portsmouth-Cardiff, that is a very long standing service and it wouldn't be good to remove it now.
Much like SWR's original plans, I would add a second semi-fast service an hour, timed to leave Southampton around xx20 which would give reasonably good spacing with both the fast and the stopper. This could call at St Denys, Woolston, Netley, Swanwick, Fareham, Cosham, Fratton, PSS, PH.
This way you get 2 fastish tph from Southampton-Portsmouth, and also 2tph at the larger intermediate stations.