Grapevine pruning query

I have a grapevine (Madeleine Sylvaner) that I've pruned into a t-shape, i.e. a main stem about 2 feet tall with two permanent arms around 6-8 feet long at that level. I have left on a couple of long new growths from the bottom, a couple going sideways from the top of the trunk and one from the top going vertically.

I've left the spare new growths on in case I wanted to replace a permanent arm, as I notice one of them doesn't seem to have many spurs. Also, I might want to lengthen the main trunk to about 8 feet and establish two extra fruiting arms at that level which is why I've left the vertical one. It's a South facing wall and the fruit would get loads of sun up there and be kept neatly out of the way of the crowded borders below.

Does this sound doable? If I use a new growth to replace a fruiting arm then I wont get any fruit off it this year, right? Can I keep both the old and new one on for this year or is that not a good idea?

It's never fruited well and I've probably had it 10 years and pruned correctly, AFAIK. Last year was the only year I got anything edible off it and I put that down to the moisture. I'm happy to have it for the Mediterranean ambience but it would be nice to have grapes, also.