I dont know if waterway plastics will do you any good or not. You can ask them. Its very easy DIY. You need a 3 way Valve of the appropriate size for your plumbing. Here is a link below to a 3 way valve.
Just cut the piple between your pump and filter and install the Valve. Then from the side port, install a PVC pipe and onto that PVC pipe, connect a backwash hose.
When you want to vacuum to waste, turn off teh pump, turn the handle on the valve and start the pump back up, and water will be diverted to waste.
Disclaimer: Link for informational purpose only. No affiliation and no recommendation intended.
Jandy 3406 Space Saver Three Port Valve 1 1/2in.-2in. Positive Seal
Regarding your algae. You need to drain enough water from your pool in order to get the CYA down.
Otherwise, you are going to have a nightmare maintaining adequate FC and keeping algae away.
If you will follow the SLAM procedure to completion, you will eliminate the algae. The SLAM is a process, it is not a one time shot. The pool stores push phosphate removers and algaecides, but they will not do the trick. There are 1000s of posts on TFP of successful slams.
Here is the link.
Pool School - SLAM - Shock Level And Maintain