Yes, 8 - 10 lbs will give you a full bodied wine. Plenty of mouth feel and aroma.
I grow my own blueberries and make 5 gallons of blueberry wine every year. I also make a muscadine/ blueberry wine that I call "Musberry wine"
Do you have a pH meter?
Blueberries are and can be very acidic. My berries range in pH from 2.8 to 3.4 each year.
I adjust my blueberry must pH with potassium bicarbonate to a 3.2 to 3.4 pH.
If the pH is lower then 3.2, you will have a difficult time getting a yeast starter to take off.
Now keep in mind, fresh blueberries that are home grown tend to be more acidic then store bought blueberries in my experience.
A cold extraction of the lavender is another excellent way to go! This would allow you to preform a bench trial with the wine.
Simply take 4 glasses of 1/4 cup of wine and add/stir the cold extraction of lavender in prescribed amounts to each glass of wine. Taste testing each glass until you find a winner.
Then make a calculation of that amount of cold press lavender in which ever glass you liked best, to determine how much of the cold press should be added to the rest of the batch of blueberry wine.