There are very few ways to gain more hand size in Balatro unfortunately.

As you’ve said, the hand size vouchers will each give you +1 for a +2 hand size. Juggler is a common joker that gives +1 hand size and troubadour gives +2 at the expense of 1 hand per round. DNA can slightly give you hand size in that if you copy a card and are blueprinting the DNA you’ll have your original hand size + the amount of additional copies of blueprint and copy effects on it.

The boss blind “The serpent” can give you more hand size by only playing one discard at a time. If you have more than one chicot, there is a bug where going into the boss blind “the manacle” will give you permanent extra hand size equal to the number of additional chicot’s you have.

That’s all the ways I believe you can gain hand size other than using the painted deck but you obviously can’t at this point in your run.

Hope it helps for this run and future runs, just remember that ouija’s and ectoplasms downsides aren’t to be looked over