There are various financial asset classes such as stocks, bonds, ETFs, REITs etc. How much REITs be more suitable for me compared to other kinds of investments? How can I make the call?

That's individual, no easy answer.

then also important also: only Singapore S-REITS or global.

S-REITS had up to the Covid crises relatively high yields on a global scale, but there is always also currency risks.

Better choice ETFs instead of single REITS.

For Singapore there are 3 S-REIT ETFs, I like thr Lion Philipp one because it has the highest true Singapore exposure.

It seems wiser however to diversify more, like with VNQ, IQQP and / or IWDP/IQQ6.