I guess you cant overshoot here with Na2CO3, so just add 50 % of your 5-MeO-Salt in weight. Then mix homogeneously until it looks as a uniform powder. Add droplets of water, if possible only 1-2 ml. This could be mixed a little bit with a small spoon or tooth pick or similar.
Then wait for it to completely dry out. Should be done overnight or at 30 °C in your oven with strong ventilation on. I have never seen any signs of Oxidation when fanning air over any Alkaloid, so I would not worry about that.
Then once completely dry, break it up, it should still be a powder (the oily Freebase Alkaloid will absorb into the Na-Oxalate powder).
Now extract with boiling Naphtha. You need to heat up the whole glas, not just the Naphtha before pouring inside or it will quickly cool down to 20 °C due to the small volume. Just use portions of ~ 5 ml and do this 3x.
In the freezer the MeO should precipitate, 15 ml will only hold back 10 mg of 5-MeO so you will not loose much. Therefore you might even do more pulls than 3 to be sure.
It seems 5-MeO can be very hard to crystallize if purity is not high enough. So be prepared that it might not work. You could create 5-MeO-DMT Benzoate in this case, which should be crystalline and still be vaporizable.