Example of a current Arbitrage opportunity:

LUNA is 10 cents on Kucoin and wrapped LUNA (WLUNA) is 60 cents on Coinbase.

Buy for 10 cents on Kucoin.
Transfer to Coinbase using the ERC20 network.
Sell for 60 cents on Coinbase.

$100 investment turns into $600 in a couple of steps.
Rinse and Repeat
$600 investment turns into $3,600 in a couple of steps.
Rinse and Repeat
....

Whenever you are satisfied you can use that $ to buy lots of XRP on Kucoin or just buy lots of Ammo

This is just an FYI and not financial advice. For experienced traders.

will UST and Luna be one of those that remain?

I dont recommend holding LUNA. Very risky. Give it a 25% chance of surviving after this mess. Arbitrage is where its at.

Is it safe to hold anything on an exchange? I have xrp on uphold that can’t be moved to a ledger until 90 days have passed. If the exchange tanks because of a market implosion, am I likely to lose everything?

Do some research on how to withdraw from uphold.

i converted mine to DASH, then i created a dash wallet on Binance where i was able to transfer to and then exchanged it for XRP again ( I found this info out by researching you tube videos on it and checking the comments, a lot of the time the comments have better info than the video)

i will never use uphold again

I am in Australia and now use only coinspot to buy as it only takes seconds to transfer to my Ledger nano wallet