Good to hear you got that pH back up. That was same problems I had with the happy frog soil. I had only used it once, and will never use it again.
Depending on the PPM you are watering with, how much nutes your plants are using, etc, you may not have to flush every 2-3 weeks. I actually only flush for the last 10-14days and don't have any problems, other than when I used the happy frog.
Once you get that PPM meter, you will be able to check the PPM of the water going in and the water coming out. This will let you know if you need to flush. If your runoff PPM is fairly close to the water going in, then there is no need to flush as the plants are using most of the nutes. If it is WAY higher than the input water, then you need to flush because there is a buildup of salts.