First you need to see how much sugar you need for 5 gal, not sure what your using to prime. Then take 10.4 % of that and thats your 2L amount. (From my conversion calculation). If your doing it per bottle just take the oz that you get and divide by amount of bottles your using.
After four weeks in that small of a new barrel you may find that the beer is too oaky. You may need to blend that back into unoaked beer. I’d probably hold off on bottling any of the batch and check on that barrel every few days to see when it is getting too oaky.
I’ve found the Brewer’s Best conditioning tablets to be inconsistent. But, that may be attributable to how I store them and the fact that the break into pieces sometimes.
I’ve been wanting to try sugar cubes as well. I am very happy with the Coopers carbonation drops for my purposes, but if sugar cubes work then I’ll definitely keep some on hand for this.
For consistency between bottles I always sterilize a pot. Then I mix my beer and (x) amount of priming sugar which I previously melted in a cup or so of water (let cool before mixing with beer). To know how much sugar I need I use the link I pasted below.