Is that default ok for most "Send" transactions? Or does it have to be adjusted for most people, most transactions, for everyone?
Usually, the default is okay. At current fees, it doesn't matter much anyway.
Transaction Fees.
There is a default recommended 0.00004978 BTC/kvb Estimated to begin confirmations within 6 blocks.
That should be fine. That would mean about 0.00001xx BTC fee for one input and 2 outputs. Let's say about $0.50.
What does this really mean?
Should one go for 4 or 2 blocks?
It depends on how long it takes for the next block to be found. As an estimate: assume there's 95% chance it will confirm within 6 blocks, probably much faster.
However:(I am willing to pay to get this done now, do not care about USD 20-50-100 transactions fees. Just want it done.)
I don't think fees are your problem. There's no need to pay USD 20 or more in transaction fees (and you should
check the transaction fee before confirming the transaction), but it looks like you have a different problem. It may help to
zapwallettxes your wallet (of which you should have several backups by now), but I haven't used that command in years so I'm not sure what the current best practice is.
Or is it better to go with Custom per kilobyte. Is there a risk with that?
Any advice appreciated.
My advice: don't bother about the fees for now.