The spreadsheet should be showing you spurs, see here:
View attachment 77695
Your actual mixer may result in low spurs, but the spreadsheet cannot predict that of course.
Also, bear in mind that the VCO signal will also have some (probably a lot!) of harmonic content.
You can filter that out of course beforehand.
I'm guessing you want to construct a test RF generator for 0-30MHz, right?
This will probably be fine, for normal use-cases. I was planning to build a sig-gen at some stage too,
but I was going to just construct many VCOs and filters, and just switch between them.
The mixer method may be better but I did not consider it for my limited frequency range need.