I agree with you on that the timeframe of 5 min should be extended to 10 min at least.

That wouldn't change much.
A manual upvote even a day after publishing a post shouldn't generate less curation rewards than an early auto-vote - simple like that.

In my point of view, that means less efforts for voters ...

Of which 'effort' are you talking? Most upvotes are automated which means no effort at all! For example a @blocktrades post shows a pending reward of about 30 dollars after only a few minutes ... Do you think anybody of theses early 'curators' makes any effort?

... they will be upvoting only the posts of their friends since no extra rewards to upvote others or to discover content.

No: they are finally free to upvote what they like, because they need not to care about their curation rewards anymore!
In my opinion the legend of the early manual content discovery is a complete myth (a very rare exception). :)

Here is another example to which absurdities the current curation system leads.

Remove the selfvoting is a nice temporary try, because eventually people will create an alternative account and delegate their stake and continue selfvoting.

That's for sure. Maybe 'diminishing returns' when upvoting the same accounts (including own one's) again and again could be worth a try.

@Mahdiyari (the hive.vote provider) should stop his service, we will have less jerky circles like @backscractchers or @honeybee. At least it will be more difficult to set up jerk circles.

Agreed!

Nope. A lot of content creators are using likwid or reward.app to liquefy rewards. The powerdown journey of 13 weeks is nice for the platform and for our security.

Did I write anything about the powerdown period?
So what exactly is your point against '75/25'?
It would be a possible measure against self-voting in case we had the suggested convergent linear author rewards curve combined with a linear curation rewards function.