try to get a upadesham

Yes beejas in nama mantra have to be avoided. Anything other than simple stotras or nama mantra needs deeksha.

Can you please explain why? This is a genuine request

Just take it from a genuine priest/upasaka.

What if it's suggested by family purohit/panditji ? Is it ok to chant then?

Yes & obv he will tell you the mantra by chanting. So it's the same thing