What are secure ways to spend lot of bitcoins on offline generated address with keys on paper wallet Suppose I have lot of bitcoins on offline generated address and private key stored on paper wallet.

Now I want to spend/sell these bitcoins. What are secure and effective ways to do this?

I will need to load the bitcoins at this offline address into some wallet (say Electrum). So do I need to load all the bitcoins, or only some? How do I do it?

So I have to EITHER sign an offline transaction made by Electrum (to send some btc to a exchange) http://docs.electrum.org/en/latest/coldstorage.html or SWEEP them to Electrum on online computer (all btc) http://docs.electrum.org/en/latest/faq.html#can-i-sweep-private-keys-from-other-bitcoin-clients

Am I right?

What the ways to spend such large number of bitcoins on offline address and private key?
As long as you have the private keys, you can spend it yourself without using any third party wallets. This can be done by signing the transaction yourself with by running a node yourself (this is however a long process and will involve downloading the entire blockchain on a machine) and once the node is running you can check out this link to learn how to make the transaction yourself.
This would be the securest way possible since you have the control over your private keys.

As far as large number of bitcoins are considered on different addresses, if you are trying to spend all of them in a single transaction, you would pay slightly higher transaction fees. However you can spend the individual inputs too if the transaction does not require a large number of bitcoins.

The method of spending bitcoins is the same irrespective of the number of inputs you are using, so the security and vulnerability does not change with the number of bitcoins involved.