I believe you don't need delay_up. It's purpose is to workaround buggy
drivers which reports link even in fact they still don't have it. If
lacp reports link is up, it is up. No delay needed.
Oh, no, that's not my primary use case.
For an activebackup two-link config (no lacp; that's what I use on my older systems, just with bonding), I use the updelay, because when a link with a higher priority comes up, it'd be suicide to instantly switch to it. The switch hasn't settled yet (especially on my older switches, which don't do Rapid STP, a switch after a reboot can be useless for almost a minute) and if my machines automatically switched the active link, all of my clusters would explode. This is a valid use case for up_delay with activebackup.
However for a multi-lacp config, using the up_delay for the same purpose is an ugly hack. What there should be is a setting like runner.agg_select_delay that guarantees a link with a higher prio gets time to settle before actually being used.