How many vias to connect a small ground zone under a small boost converter ?

[microvia]

About the specifics, you are overthinking all this.  No need for star grounds.  No need to worry about EMI at all.  You are never going to get FCC/CE tested and you won't have the test equipment to measure it, so don't sweat it.  Just select a part that has a recommended layout and copy that.  TI is really good about this (Yes I know everyone rips app notes and recommended layouts and such, but it's a significantly better starting point that this).

No need to worry about EMI with a boost converter IC not far from an RF chip ? I made a special test pcb with four different boost converter ICs, three of which used the same type of inductor and capacitors. Of course I selected a suitable shielded inductor, and capacitors with derating and proper size. Two ICs significantly altered the RF operation of the target circuit. A third one made it work ok but not as good as expected. The last one gave satisfactory results.

[microvia]

Just a side subject question.
I always apply 2 vias directly under the tab of 1206 smd capacitors.
I see this is avoided here, is it a bad idea?

It's not avoided, but as some PCBAs charge more for that, I must consider the best option.
Of course, if I use JLCPCB's free vias-in-pad, I will update the layout accordingly.

[Kokoriantz]

Thanks, luckily, I only deal with JLCPCB. 

[microvia]

Here's the updated layout around the boost converter IC with improved ground path.
The real question is whether anything can be improved, especially if I put too many vias. I can add one under the bottom capacitor.