Please Help: beginner with crashed tank and many deaths

Our tank is less than 3 months old and has been doing great with 6 glo tetras and 4 nerite snails.

I added too many root tabs and the pH dropped very low. I took a water sample from my tank and faucet to LFS. The tank was at 6.2 and my faucet at 7.2. They said not to add or change anything and just let the tank sit since it’s still considered a new tank. He said to change 20% of water per week and continue to vacuum. He was surprised nobody in the tank had died and told me to expect for at least the snails to die. I asked if there was a way to prevent that from happening and he just sort of shrugged his shoulders and said it was a reality of keeping an aquarium.

We have since lost our smallest fish (although I think this may be due to other reasons, as reviewing other tetra posts makes me think he was a different type of fish since his size and body were so different from the others).

Two days ago I found two of the snails had died. The pH is still low and now the kh is off too. I purchased a tetra ph balancer and added it yesterday. Today, I found a nerite on its back and the helmet snail was loosely attached to the glass.

I pulled them out of the tank and clarified some tap water and put them there - my tap water tested healthier than the tank water.

I’m just now sure what to do at this point. I’m so tired of feeling like a murderer - my post history will show that we’ve lost 6 snails at this point and it’s all because of my ignorance as a fish keeper. I’m feeling immense guilt and also super tired of explaining to my 5 year old that another pet as died.

At this point, do I drain the tank and start over? I added more plants to try to help the environment but feel like this led to the crash.