Help, Size of Return Lines to use.

Helping a buddy redo his 21,000 concrete inground pool. Removed all the concrete decking around it and old plumbing and skimmers, there where all busted from years of neglect. Had 2 skimmers, 5 returns and a bottom drain. Originally each item had its own line running into the pool house. Plan on putting back all as well, plan on running each all the way to pool house so if issues like a leak can close the valve and plug. I was thinking spend a little extra and run all lines as 2" and just size it down at the returns, for less friction loss. Is that a bad idea. Or would 1 1/2" be sufficient as each line is seperate. Some will be running probably 50' long.

Also would using two 45 fittings be more efficient than one 90 fitting.

I believe in general, the bigger the pipe the better. If you're replacing pipes anyway, go with the bigger ones.

I just replumbed part of my pool. 1 drain and 2 skimmers as well as a single return all 1 1/2 inch. I repiped the return with 2 inch and got a significant reduction in head loss. But in your case you have 3 supply lines to the pump and 5 returns. The amount of water in any one pipe is far less in terms of gallons per minute than in a single return setup like mine. Still, bigger is better and two 45 bends (or a sweep) is better than a 90. The $$$ difference is probably minimal.