No advantage doing a diploma,
But do give a bit of thought to a marketable degree. I fondly recall Ralph Caponi in Melbourne. Originally an ANA pilot back in the day, started his GP quals while still flying, ended up a GP, then qualified as an electrical engineer, possibly a LAME as well (?) and ended up running an EI&R business at Essendon while playing at being a GP in his spare time.
Flying is great fun, but it is a bit tenuous in the long term due to the medical fitness link. Not nice to be out on your ear mid-career and nothing much else to fall back on.
University degrees have no value.
Maybe yes or maybe no. I got into Ansett largely on the strength of my engineering background. My flying experience was minimal (near invisible ?) when compared to that of my intake colleagues. PhDs in basket weaving possibly don't offer much in the way of marketable valued added attraction ...