what is the best way to store medium-large amount of ONIONs?
Hold them in your runnig wallet and get 5% of staking rewards. That should be a good reason to run your own full-node isn't it?

Remember that 5% will be available until July 12, and after that, the reward will drop to 1%
Knowing that (and I have known about that behavior) every single day is counting! Let your wallet run 24/7 and receive your 5% staking reward until July