Really? Tsp? I never heard that, but I'm willing to try anything and I seem to have multiple problems.Im guessing that when mixing in the added peat additional lime was needed to buffer the ph back up into the 6’s. The organic potting soil was likely buffered but the amount of lime it had is dilluted now.
Im sorry i dont know how to advise how much line to mix in as top dress but can be done. Go easy is always good advice.
Also maybe using tap water would be better than ro for the calcium content to help keep the ph up in the soil.
The seedlings look like their soil is acidic for sure.