First: Concentrated sulfuric acid is not completely harmless, be sure to wear goggles and gloves during the activity.
The sulfuric acid and the isopropanol must be as free of water as possible.
If you use pH paper, before testing the freebase solution, wet the pH paper with distilled water, this will guarantee an accurate result.
I would take a smaller amount for testing.
For example, mix 10ml of your possible freebase (A-oil) with 30ml of isopropanol.
Then mix the acid solution with 2ml of sulfuric acid and 10ml of isopropanol. Here add the sulfuric acid slowly to the isopropanol, be careful it gets hot. Use a syringe. If the amount is not enough, simply mix a new acid solution and continue.
The freebase (A-oil) is basic with a pH greater than or equal to 10, with the acid we neutralize the base to a maximum of pH6 (neutral), anything below this would reduce the yield and the sulfate will turn reddish.
So once you reach pH 7-8 proceed carefully, and always mix the freebase solution well after adding the acid, as I said before, if it becomes too thick add more isopropanol, even if it should be 100ml
Separate the obtained sulfate with a filter and dry well.
I would do one more two cleaning steps depending on the situation, if the sulfate is clean you can consume it immediately. Caution very pure