IMO you’re not going to go from unyielding to comfortable with a change of forks. The differences are more subtle than that. The 853 V brake fork I have is very nice and I like the look of it. But it isn’t hugely different in feel to the hi-ten one on my hybrid and that complete bike cost less than the Thorn fork! I wouldn’t run the hybrid on high pressure 28 mm tyres, but there’s no need to
Apologies if it seems rude, but if you’re finding the Mercury uncomfortable, it’s likely to be something other than the fork - tyres, pressure, bar height, riding position...
If you really do want to swap forks- then a carbon steerer if yours is alu will make a small difference and lighter will have a bit more give, though the flex you can build into carbon is limited. I test rode a Spa Elan which was a comfortable bike, you could check what fork they use, I haven’t ridden the Thorn ones to compare. There’s a few options in steel, including Thorns own, I haven’t ridden any of them, though I’d be surprised if the difference was ever more than going up a tyre size and down a few psi
