Cheap Kneel-up paddles

- so that you can try it

Your SUP paddle will not do for kneeling up unless it is a retractable one and fully retracted. The problem is your upper hand is not doing its job. Holding a tube is very different to holding a handle that tells you which way the blade is facing. The handle also allows you more force in thrusting the paddle into the water. The correct length for the paddle is approximately 1 to 1.5 meters (3 to 5 ft).
Shop around in your marine shop for items like these two for under AUS$30:

Alloy 4' Curved Blade T Paddle

.. aluminium shaft, polypropylene handle and blade. Overall length 1.20m (4')

whitworths.com.au/main_itemdetail.asp...&search123=55338

You can fine tune the length by cutting bits of the plastic off the end of the blade.

The length you need will allow the blade to be clear of the water (with your hand holding the handle) when you reach out to put the blade in the water at the start of the stroke.
OR if you want something more hipster:

High Grade Timber Paddle

.. all varnished timber paddle with comfortable hand grip and strong blade.

whitworths.com.au/main_itemdetail.asp?item=55459
This length is good if your board sinks a bit when it is not yet moving forward. The handle is flat enough so that you know which way the blade is facing.  What you don't want is a paddle with a round knobby handle that does not give you that idea of which way the blade is facing. If you want to spend more than that search for "dragon boat paddles" and look at the cool-looking range available.