Thinking long term, say 25 years from now, and assuming Network Rail’s demand forecasts are accurate, would tram train be able to provide enough capacity to meet peak time demand on the Cathcart lines? Could you feasibly run 30-40 tram trains per hour in the peak?
There’s a high risk you’d regret giving up the potential capacity which heavy rail would offer.
A necessary part of metro conversion would be a shift in expectations. In the peak someone joining at Pollokshields might have a tram every couple of minutes but they're likely going to have to stand up for the few minutes it'll practically take them to reach their destination. Most people around the world wouldn't bat an eyelid at having to stand for that sort of journey. The fact that they might currently have more seats and other facilities is an anomaly.
This discussion has been had before, but the theoretical capacity of heavy rail is just that - theoretical. Unless the economics are there to actually reach that capacity, it isn't actually realisable. The need to shift a vast number of people during the peak doesn't always mean it's economical to do so as that capacity could well sit idle the rest of the time, losing money. A system could be better overall despite having a lower theoretical peak capacity because of how it better matches demand throughout the rest of the day.
I mean, if the cost savings/benefits from using metro light rail are as high as they appear to be out of the peak, it might be cheaper just to throw more bus services at Pollokshaws Road for a few hours during the peak. Rail is just one part of the transport system and deficiencies in one mode should be made up for by other modes. Buses are much cheaper and more adaptable to demand changes than rail is.
Another option might be 5x23 units. Would have more space as you only have 2 cabs not 4 and only need 1 accessible loo instead of 2 and still fit in the exisiting platforms.
The train frequency through both tunnels is very low at 6 an hour via Central and 8 an hour via Queen Street so another option would be more trains rather than longer ones, although some of these would need to terminate before the routes merge at Partick.
There's a cheap solution for more capacity on either North Clyde or Argyle line networks east of the city - finish building the third platform at Hyndland. The fact that it hasn't happened yet, and isn't planned either, is possibly indicative of the lack of any need to do it. What would the extra services actually be? A major constraint is that the Yoker loop is the only real overtaking opportunity on the whole network. Extra services could only fill certain gaps in the timetable rather than enabling fundamentally new journey opportunities.