Awesome, thanks everyone for your kind help. I have just made a new batch of cucumbers (1.5L jar, holding 750g baby cucumbers), this time, I used 1.5% salt to total weight (equivalent to 3.4% salt to water weight) + 35g sugar + 1 Tbsp lemon juice (per litre) + 1/4 tsp cayenne pepper (per litre).
I have boiled the water and allowed to cool in the refrigerator until cold before pouring the brine in. Regarding the salt amount, I am well aware that 5% salt to water weight (equivalent to 2.23% salt to total weight) is typical, however, my mum is very sensitive to salt, and so it was important for me to keep it at the same level that is in sauerkraut.
Let's see what happens in 2 weeks time!
If I get mold again, I'm buying a vacuum fermenting lid.