Personally I use Boundless Ages and a Serendipitous Assault. The on demand burst is a nice addition, and you can time it with your SA proc if you really feel like it to get a few extra seconds of higher power. The mending relic doesn't help much, and when it does proc it often seems like its wasted on overhealing.
From what I hear, SA relics of different tiers are stacking procs atm. I personally wouldn't use two SA relics while healing because with my luck I always get the procs when I don't need them and they are wasted.
But find what works best for you, maybe you are lucky and can get it to work.