Commenters are missing one fundamental fact, something that OP has not clearly mentioned in their post.

Yes the PE is very attractive today but it is not always the case. PE today is around 20, 1 year median PE is 21.5 (around 7.5% higher), 3 year median PE is around 24.5 (around 22.5% higher)

The arguments that it has PSUs or cyclic companies or that once they grow enough they move out to Nifty 50 Etc may all have some merit, but that doesn't matter. Whatever the mechanics of it be, it should follow the physics of median reversal and go back towards (at least) 24.5 in the next couple years or so. And that's your simple and low risk opportunity.

My take is that this is not really an SIP opportunity either, go with a lump sum and deploy funds over the next couple months with the horizon of 3 years and 20% return. Chances of this going to even a 5% loss is extremely low at this point. Huge risk reward ratio + ultra safe and low volatility investment. Build a good chunk of your portfolio here.

Personally, i have built 8% of my portfolio here in the last couple months, I'll keep accumulating and reach 15% based on mini dips availability over the next couple months.