no clear indication when exactly they will be deleted for being unused
I think you're going at this the wrong way: source Merit doesn't expire, but it stops replenishing once you're at your maximum source amount.
If you want to maximize your source supply, you'll have to empty it as fast as possible, so that it gets replenished again.
(Merit Source’s merits are not all refilled on the same day after 30 days, but rather 30 days after each batch is sent, even down to the same time and minute).
Correct. Let me give an example:
Say you have a total source supply of 10 sMerit. If you spend 1 per week, you will always have about 6 sMerit left and send a total of 52 sMerit per year.
If you use your sMerit as quickly as possible, your source supply will be empty most of the time, and you can send about 120 sMerit per year.
I try to keep my source amount as low as possible, but haven't been able to "stabilize" that yet since it was increased at the beginning of January. I now get a large and fast replenishment at the start of the month, and empty it later on. Eventually this will average out.
At the beginning of this month, I had >25% of my source available, it's now <10%.