Add circulating pump for instant hot water and other stuff

My only comment to your neat idea is to be very careful about how you seal your fittings.
In my experience as a pipe fitter, do not use teflon tape on any plastic fitting. Use pipe sealer. The tape will cause the fittings to develop stress cracks because it acts as a wedge in between the threads. The sealer will squeeze out and not split your fitting assembly. Teflon based sealers are great.