I think there are 2 main considerations:
1) Will you have enough evidence for a de-facto relationship?
2) What if the new sponsorship rules come into place by the time you are ready to apply for a 820/801 visa? This would require the sponsorship to be approved in the 3 month period of your tourist visa, before a visa application can be lodged.
By applying for a sc. 300 visa now, you will have a head-start of about 7 months.
It may also be wothwhile having a look at skilled visas, but I assume you have already done that.
Nick van Voorst
Registered Migration Agent 0640648
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Please note that this posting is of a general nature only. It does not constitute legal or migration advice and may not apply to your particular circumstances.