I'm a 33 yr old female, Australian and my 28 yr old male US citizen fiance is petitioning me with a K1-Visa

In 2018, from January to August, he was self-employed with Uber and Lyft as a rideshare driver. Then, from September to present he has been employed full time as an administrative assistant. 

Gross income: 
2014 = $8,458.  
2015 = $10,715. 
2016 = $1,651. 
2017 = $16,093. 
2018 = $23,928. 

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2018 is the only year that he's been above the poverty line for the minimum income requirement (He lives in California). 
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Should we have his father sign as the co-sponsor? (His Dad has worked full time managing a restaurant since 1980) 
Will a co-sponsor even be sufficient enough, due to my history of low income?
What are your recommendations to have the strongest and most successful application in this scenario? 
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Note: He has not yet submitted his 2018 tax returns. If you have any recommendations on how to ethically make the tax return more favorable for the k1-visa application please let us know.