Jamesh wrote: ↑7 Aug 2022, 10:30pm
Might be a bit far for you but I cycled around Llyn Brianne Dam & Reservoir but could only go half way due to being on a road bike but it looks like you can do a full circumnavigation on gravel bikes?
I do have a "'round Lynn Brianne" ride plotted on OS Maps, which tells me that it's 98km and 1899M of climbing from my front door back to the same door later on, when I'll be a shadow of my former self.

Such a ride is within my current capabilities although it's about 2X as far as my habitual rides, which are often of the same climb rate (around 1000M every 45-50K).
A cafe at Tregaron might be necessary. Or I could stuff that pannier with cake. The last third of the ride is mostly down the gradual downhill from Tregaron to Lampeter, although there is a steep 100M climb out of Lampeter on the final leg home at the end. All it needs is a following wind and perhaps the ladywife will go too. A riding partner increases the pleasure and decreases the fatigue, I find.
The route I've plotted is all on tarmac roads - although some of them feel like gravel roads once one gets into the Cambrians proper, as I know from various forays around Ffarmers in the Twrch valley. However, it is possible to make a shorter circuit of this route by taking a gravel road over the tops and down into Llanddewi Brefi, missing out the dog-leg to Tregaron that the road route takes.
Persuading the ladywife (my only riding partner just now) to go on such a ride is beyond me at the moment. In the four years since she's taken up cycling instead of running, I've got her up from 25km to 50km with all the hills. She regards longer rides as self-abuse, just yet ... but I'm working on her! A lack of sufficient cafes in this area is part of the problem, as a halfway stop and fuel makes a big difference to one's mental stamina when it comes to the longer rides.
Cugel
“Practical men who believe themselves to be quite exempt from any intellectual influence are usually the slaves of some defunct economist”.
John Maynard Keynes