I got a strong boost (~10k views/day for 3–4 days)
Did you look up where those views came from in your analytics page?
To trigger a boost
There is no such thing as a direct boost. Itch does not promote games in most cases. There is a bit of attention by being on page 1 of recent games in your category. But you have more than 1k followers and had over 40k views when your game was new. It is a special nieche. Chances are that most of the people that are interested in that combination and regularly look at recent already have viewed your game's page, even if your second "major" update would have been approved. Also, if you do select some tags, you are still on page 1 of recent.
Is there any cooldown between two approved Major Updates
Most certainly. My assumption is, that some criteria will not even put a devlog in the queue for consideration of being approved to reduce workload for staff. And then one can assume that since those devlogs need approval, that there is a finite amount of approved devlogs that is independent of submitted devlogs, but dependent on staff time. And staff time is bound to be influenced by external factors like sick days, ddos attacks and who knows what. And of course, by working hours. Your devlog is 15 hours old.
Plus of course, they do not approve all devlogs, else they would not need to have that mechanism in the first place. Not actually updating files might be one of those factors that might reject a devlog by automatic.
Eliglible is a vague attribute in context. And if those guidelines about eligility would be enforced strictly to the letter, no, demos that just link to another store would not be eliglible in my opinion https://itch.io/docs/creators/quality-guidelines#avoid-only-uploading-keys-or-li...
But those guidelines are guidelines and not terms of service dealbreakers. Also, the situation for adult games is not solved.
Anyway, your game is indexed, and your delog appears in https://itch.io/feed?filter=posts if you scroll down.