There's a different, two-part way to approach that general idea:
Part 1: No new restrictions on new users, but existing users (either everyone, users with X posts, or paid users) can toggle a filter that applies to all forum views / feeds (mechanically the same way as the ignore list). When activated it remains active for 24 hours, but can also be manually turned off. While active the filter hides posts from A) accounts less than X hours old (say, 24) and B) accounts with fewer than Y posts (say, 50).
Part 2: That relatively high post requirement means posting that much within a short period of time can be automatically detected as "spamming" whether it's a new or existing account, and the user's ability to post can be temporarily revoked if spammers try to work around that limit. Spammers by their nature aren't going to create accounts and take days or weeks to contribute meaningfully to discussions (and rack up post count) just to be able to then use that account to spam for a couple of hours before getting suspended. Meanwhile automated, nonsensical filler posts, even if drip-fed, will get reported by users as spam, just as before.
This approach doesn't hinder regular new users and allows normal interaction with them most of the time; doesn't add extra burden on the mods; gives existing users a way to easily and temporarily neuter spammers until moderators are available; and spammers have no cheap and effective way around it.
No idea if any of that is technically feasible on this forum software, or worth the MR team's time and effort, of course.