I’ve only got a tiny amount of experience, but your weights sound low for Saddleback crosses.
My rule of thumb was one pound of food per day per month of age, up to four pounds, then stick at four pounds. Sow rolls from four months on, grower pellets until three months ish, introducing sow rolls gradually. As much fresh veg and fruit as I could get for them.
Backfats higher than yours but not excessive. Butcher impressed with 5 month old Saddleback x OSB deadweight 54 kgs backfat 12mm, said he’d buy them off me if I could keep the back fat at that level but get them a bit bigger. (That pig slaughtered a month earlier than I’d otherwise have done as I took half the carcase on a butchery course
).
My slowest growers were the Large Blacks, took over 8 months to get to 60kgs dw, but backfat resonable (15mmish I think) and flavour amazing.
On my very limited experience and knowledge, I’d expect Saddleback cross MW or Welsh to get to 60kgs dw by 6-7 months no bother - but with more backfat than yours, so probably been fed more per day.
Oh, and I did reduce rations for a week or two before sending off, to reduce backfat. I managed weight by eye and feel, really - I like to be able to see where the ribs are, but not actual ribs.
If they’re outdoor all year and you’re rearing through winter too, making sure they have plenty of dry bedding will also have an effect. I was horrified to discover how wet the straw in the ark got through them coming in wet and muddy, so took to lobbing in a few flaps of straw to the back of the ark every day, and forking out wetter stuff to in front of the ark, which gradually built up to a pad of less muddy ground that helped keep them drier on entry too.
If they’re wet and cold, it will keep the back fat down but also keep the growth rate down.
Again, I’m saying all this on not much experience at all, but maybe there’s something useful in there for you.