Difficult to answer, but you can certainly increase your range a lot with an 8 m2 and two linesets 25 and 30 meter yes.
I got "loads" of kites, so usually take a bigger kite and use my "normal" 26 meter lines.
Only when around or sub 10 knots, or changing winds, I go to 30-31 meter lines with a 9 or 12 m2 kite.
Your 13 m2 edge with long lines will actually let you ride in 8 knots, but of course, if it hits the drink you have to drift ashore, and it can happen quite easy in this wind - but it is totally worth it, I think you will find out in time...
If a limited budget, I would use extensions and not two bars.
Assume you ride waveboards since you aim for a 1 or 2 kite wavekite quiver.
So it depends on your conditions IMO, as for waveboards a 9 and a 7 could work fine, and also for hydrofoiling down to 9 or 10 knots.