Well, that is the all depending of your budget I guess, all of these mainboards are proven worthily in the mining field s there are not so many differences between the mainboards.
But.. for me.. better choice a simple h or z 270 mb + some 1x to 4x pcie -e riser and that is all

Maybe it is overheating? I mean the spring is there, outside temperatures  are rising and your home temperature is higher as well and that might cause the hash rate drops  and are you sure you windows have not auto updated the drivers?

I think that would be really great.  But it would be very easy and that is why I do not think so it will go at this scenario. should be different, then last year. So it might be go down more  and more.
Then go flat. And maybe, yes, maybe after two years will start to climb up again.
Who knows..

There were a lot of rumour about such actions. that all ASICs manufacturer before delivering batches mine some coins and then dispatch the shipment. Sadly there is no proofs, but some miners have spotted diff spikes right after news modes of Asics announcements..

I think in some terms bitcoin is clearly better then gold,  for example bitcoins can be stored or traded by general public, while gold ( not jewelry)  is prohibited to trade in many countries..
in other way, gold will not disappear if internet is gone, while bitcoin would.

as far as I remember NiceHash paying once per week basis and btc wallet on hitbtc is ok, cause it is just a btc wallet, you can use earned btc on trading on that exchange if you want..

In my country the electricity cost is about 3.1 cents per kw.
So right now mining is still profitable, but at least 5 times less profitable then January, which makes me very sad

I like bitcoin because Bitcoin make me good profit.
It good like investment. It has a great technology.
It forms a more independent and fair system of crypto.I believe in Bitcoin.

Well it might be bad connection to the card of power cables from your cpu, or dying riser, or maybe some dust cause it.
Check everything, all connections, remove and insert the card from riser.. and observe any change

Maybe you should go for different setup? like one psu will power your mb, cpu, ssd, all risers, while other psu will power all gpu cards? actually all of my rigs are working by this sheme and I've never had problems as you described

Let's imagine, that someone here knows the answer, why he should share that kind of knowledge here? ))
But if we can assume no one knows, then the answer to the question you've asked is a pure speculation.
I do not know how much it would be, all I hope it would at least twice more then now))

Well you can order the fans from alixexpress or google, check this one
https://www.aliexpress.com/item/Everflow-75mm-T128010su-12v-Cooling-Fan-Gigabyte-GTX1050-1060-1070-1080-G1-N960-N970-Graphics-Card/32826852544.html?spm=2114.search0104.3.1.49db7019kXaQJ6&ws_ab_test=searchweb0_0,searchweb201602_2_10152_10065_10151_10344_10068_10342_10325_10343_10546_10340_10548_10341_10696_10084_5722520_10083_10618_10307_5711220_10059_5722620_5722920_10313_10534_5722720_5722820_100031_10103_441_10624_10623_10622_10621_10620_10811-5722620,searchweb201603_50,ppcSwitch_3&algo_expid=eb047381-6d5e-4027-8d8c-d7f6ec35159e-0&algo_pvid=eb047381-6d5e-4027-8d8c-d7f6ec35159e&priceBeautifyAB=0
Shoud be ok, i think

Yes, sometimes that noise happens, it is the coils in 90% cases. it is not realy dangerouse I think, cause manufactures says that is normal.. so I guess it normal then.  you do not do something special with your cards, a lot of people mine zec with your setting and their cards are doing just fine.. so yours should be ok too

I think gtx 1070 would be better choice for nicehash, because it can be mine more algos with higher speed then am rx 580.. yes, seems the price for gtx 1070 is more now, but in a long term it is better imho

No, i disagree.
I think now transaction of bitcoin are way faster and cheaper then it was, say last year autumn.
And if you check the mempool you will see it is empty, that is why they are fast

If your choice is a new card then I think GIGABYTE GeForce GTX 1070 Ti GAMING 8GB is a good option
but, try to check some used cards locally with some active warranties - for example Asus have 3 years warranty,  so you can source asus strix cards with 2 years warranty left with much better price

Sorry mate, cant advise you here on your question, because I'm using for my rigs old home made design from wood) 
So far it was proven actually quite well. durable. cheap. can be replicated and repaired if needed, I'm quite ok with that

You say BTC fell down from 20k, but I think BTC had grew from 1k in the beginning of 2017 to 7k in the beginning of 2018.
On topic - I think cartel affects on bitcoin. It controls gold, uranium, oil and wants to control and influence bitcoin

In order to help you, more information about your rig is required. What mb do you have, what gpus, what psu and so on.
Please update thread with details

Are you seriously expecting to receive an answer to that question? 
Profits are low these days, and I'm afraid no one will share coins with good profits.
You have to search yourself