Two cents from a middle ager ... I transitioned out of shortboarding due to insufficient fitness, just not strong enough paddling to compete in the crowds. Longboard, sup, kitesurf, supfoil, and now full circle to riding sub 5 foot boards prone foiling in my 50s. I suspect I could now get away with riding a shortboard again in decent waves, as I am probably in better paddling condition than in my late 30s. Which is kinda sad looking back on it.

I started prone foiling bigger higher volume boards (converted surfboards) and steadily chipped away, but once I got down to somewhere low 30s, I found the paddling too much of a slog. I could catch waves for a short time, but would use up all my energy plowing water paddling back. I have lately been using short n fat style, 4.6 board but 4" thick, so probably 40ish litres. Its works for me, no problems paddling or catching waves. People talk about feeling too disconnected from foil on thick boards, and that may be, I plan to experiment again with thinner to see if it's a noticable difference. Another option is to cut in some deep concave on deck, same deep waterline but slightly closer to foil.
Another buddy, same age as me, similar surfing ability, has passed me in foiling capability...he's achieved pump monkey status, while I am struggling with that. His main board is an older blue planet 5.5ish probably 50L and he RIPS on it, getting multiples and big carves. I bring this up because if you believe what you read online, he should not be able to do what he's doing -- but there he is doing it. So I think take some of the board performance obsession with the old grain of salt....it reminds me of endless debates back in the 90s about adding 1/32" extra width on our nearly identical potato chip thrusters that we could barely paddle. :-)