The usual big reason NOT to max as soon as possible is to get any employer matching funds. Sounds like you've already looked into that and it's not an issue.
The argument in favor of maxing as soon as possible is that on average the markets go up over time, so the longer the money is in the market the better (again, on average).
For me, just for the sake of simplicity I would prefer to just set it and forget it, so I'd probably try to figure it out so I "made up" for the part of the year of that she'll miss, and then reduce it down to $1500/month (or the equivalent depending on her pay cycle) and then just leave it there. If we can assume that any extra income in the household will be invested anyway, then I would be even more in favor of this option since you'll be investing it as soon as you have it anyway so that just means it might be in a tax advantaged account a bit longer, but otherwise the returns should be very similar.
That said, the absolute most optimal strategy, ignoring the simplicity factor, probably is to max it as soon as you can.