I was reading up on sendmany. Does it have to be done from Bitcoin Core, or could it be done from a GUI type wallet, and if so, where is it!?
most wallets do support adding more than one recipient to your transaction one way or another. for example in Electrum all you have to do is to choose "Pay to many" option from Tools menu and then entering one address with its amount per line in the "Pay to" field.
Also, I'm not clear on the fee savings of using sendmany. You're sending more bytes because you have multiple addresses, so how does one estimate the cost/savings of actually using the sendmany function vs sending each one separately?
each transaction has two parts, inputs and outputs.
when you sent to many in one transaction, you don't have to repeatedly create new tx and have one or more inputs per tx. instead there is for example one input and 100 outputs compared to 100 transaction each having 1 input and 2 outputs (total 100 inputs and 200 outputs). and an input (tx_in) is the bigger part of a transaction because it contains the signature.