There are a bunch of options for high interest from crypto lending.
I am biased about third party lending and I prefer p2p lending as a good source of a smallcap business, I ran one such thread in another crypto forum as well.
1. Since you have to give possession of the coins to the lending company, they can be hacked and you can lose everything (I understand Celsius has insurance, what does it cover?)
I can tell without even looking at the terms and conditions, that the insurance will rarely be enough to cover the loss. In case of a hack, as we have seen on many exchanges and so on, the money is lost for good and nothing can be approached legally. It is a downside of these smart contract lending services.
2. The borrower collateral will be worth nothing after a crash and you will not get the interest you are promised.
Considering lack of collateral liquidity to be a normal risk involved in lending, this is a common thing but to prevent or possibly reduce this, lenders prefer to take a higher value of the collateral (120-150% of loan value) and that too top 10 altcoins. A more diligent lender will also do a thorough research on that altcoin for possible backdoors and hackable stuff in their blockchain to prevent a theft.
So are they all too much of a risk?
Both type of lending have pros and cons.
What happens after a crypto crash?
What would happen to fiat markets if they crashed? Simple answer. But if you keep thinking in the lines of "world ends" you will never be able to do anything fruitful.
Or are any of them safe because they collect a lot more collateral from borrowers than is needed?
I dont think they collect too much collateral, rather the regular amounts. Have you been a borrower yet?