Not to be a negative nancy because I do love them, but I would add that not all cobs are lovely calm ploddy totally reliable types just like not all TBs/WBs are hot, wired, spooky - probably a greater proportion of cobs fall into the bombproof category than nutters, but I have owned 2 that definitely fell into the high alert at all times, spinning eyes on stalks if they saw a crisp packet or similar, constant tantrum negotiation category (TBF I got them both knowing they'd had relatively bad starts in life and were very far from plods. Plus they probably had some Welsh blood somewhere which IMO adds 'personality' for sure)! And to make it worse one of mine was very unsound in all the ways you'd expect from a much higher-powered horse, hocks, back and ultimately ligaments which was the final straw for her ridden career 
. I guess what I'm trying to say is not to be down on cobs per se, but I think it's the individual horse that gives you the happy, safe feeling not the type/breed, and so if your friend's boy is giving you that now, and you can ride him often enough for your liking, I wouldn't be rushing to change it.
My experience is similar, after close on 10 years of pretty much constant hard work and heartbreak with the aforementioned 2 WB-alike cobs, I'm now loaning a friend's late-teens connie who is winding down for retirement so just needs to be ticked over a few times a week, like your cob he's wonderfully reliable and safe and has taken me from a bit of a confidence dived, frankly quite poor rider (because I'd learnt to be so cautious and defensive all the time) to happily whizzing around a local clear-round SJ, yes both me and the pony are fatter and less fit than we should be and it's a comedown from both our competitive heydays but the important thing is we have a smile on both our faces! Plus after so many years of enormous vet bills and never knowing whether I'd have a sound horse from day to day it's very reassuring that his owner retains ultimate responsibility for him and has promised him a home for life once he's ready to stop work, what to do with the old or broken-down ones has always been a major headache of horse ownership for me so it's great not to have that worry any more esp as I get older and contemplate my own retirement!
On the other hand, I do sometimes feel a bit wistful that current boy isn't truly mine so I do get the draw back to ownership - maybe you could look around the market in a very casual way and see if a horse does give you 'that' feeling, but without feeling pressured to find something?