I'd be in two minds on that one. A section of this route south of Trawsfynydd is already a path, one of the best railway paths to be found, and part of the reason for that is it's *not* particularly well known, nor waymarked as a cycle trail. In all honesty I'd prefer to keep it that way.
Some ex railway cycleways are too busy nowadays, it's hardly a relaxing experience to hear "bing" every few seconds, which is now the typical experience on something like the Monsal Trail.
That just feels a bit elitist to me, perhaps a bit of 'I don't want anyone using this route but me'. Which is understandable to some extent, it is nice to have a nice bit of scenery that isn't well used, even I like to find a path/road not heavily used but fun to cycle along. I have a number of such favourites around here.
To not want others to visit and enjoy the same beauty as you do already, that's not good. Yes I understand it must be frustrating, I get frustrated by groups of dog walkers on a sector nearby that seem to get in the way on purpose, or families that take up the whole width of the path. Said frustration usually disappears very quickly, especially when it dawns on me that they are also out for the same purpose as I am. That is, to get out and about in the fresh air and away from the traffic with all the dangers it brings.
Would I try my best to keep a favourite route hidden? I would consider it, no doubt, but not for long as I would also much more strongly think about how much fun it would be to share the route and the experience with others.
There is an extreme amount of natural beauty in the world, and our country is fortunate enough to have a lot of it. We should celebrate this, and share the news about a really nice area to explore with others.
As for the Monsal Trail, yes it is no doubt often busy but is that a bad thing? This clearly shows it is popular, I haven't been myself yet, but it will happen in due course. I am fortunate I get to go in the middle of the week, when the schools and so forth are in session, so I can enjoy it when it is quieter. Not everyone is as blessed as I am with that possibility, so I understand it must be annoying if you're hearing bells every few minutes. My view then is what would you do if it wasn't popular, and well supported, and left to go to ruin as many paths do when not looked after? As it is a popular route, it is no doubt well cared for, which makes for a better experience for everyone. If this was to change, and no-one cared enough to look after it and it was not well used as a result, what would then be your concern? I would say it is likely that you would then be concerned at the state of the route and want it kept in good condition.
I hope you see what I'm saying, that trying to keep your preferred route in North Wales a secret, and it's going well because even when I looked at the North Wales map yesterday I didn't see such a route available, well it's just not good for humanity as a whole. I'd love others to come down and experience my local routes, there's some really nice areas to explore!
So, closed railways I'd like to convert to a cycle route. Despite it being in sparsely populated territory, I'd love to see the old railway formation between New Radnor, Kington, Leominster and Bromyard (even to Worcester actually too) with a cycle route in its place. It would be well used for leisure cycling, not really much at all for commuting and, unfortunately, there's significant changes to the old formation so it could never re-open fully and absolutely never as a railway. A shared-use path would do in some places, such as Kington where the bypass takes over a fair chunk of route. Similar could be done at New Radnor (the terminus of the railway, it was meant to continue west but it never did), the site is now a caravan park. The rest of the route I'm not too sure on these days, I don't know what's happening at Titley Junction now as once upon a time they were looking to build a heritage railway up there.
Other local choices would be Hereford to Hay on Wye. Another sparsely populated route, and the formation I can't comment on as I've forgot some of the details, but no chance of a railway there again. No commuting available as a sensible option, and the route out of Hereford has been redeveloped in multiple places, same in Credenhill.
I can't think of any others right now, I'm actually due to get up in around 5 hours time to go exploring an old railway amongst other things so I'd best get some sleep!