When I go to "Send" tab, and enter a target address and amount and click on "Pay" button
then a popup appears which suggests a mining fee e.g. 1.8sat/vbyte for a execution within 25 blocks

Who pays this mining fee? the sender or receiver?
I don't want that the receiver pays any fee.

As far as I can see the minimum suggested target fee is the one for "within 25 blocks". ETA
That means approx 25 * ~10minutes = ~250 minutes = ~4 hours.

What if I would accept a transaction within lets say 1 day =~ 144 blocks

Getting a suggested mining fee for that period seems to be not possible.

I remember (at least in former times) that even a mining fee of 0 leads practically to a execution with 1 or 2 days.
That no longer seems realistic, does it?