I can see where you are coming from by keeping the 26” Oracle as a reward for meeting your goals, but given you are learning on rough ground you might actually achieve your goals faster if you learn on it rather than your 16”. With your 16” small stones and undulations in the ground can easily throw you off whereas the larger wheel could roll over them. There are disadvantages from going straight to a larger wheel and it is generally advised to start on a 20” on a smooth surface, but a 16” is maybe making things a bit harder than they could be.
I learned on a 24” on roughish ground, that might be a good compromise if you can find a secondhand one and keep your 26” Oracle as the ‘prize’….