My concerns:
1. Do I need amplifiers/dsp for the four speakers?
you need to check if your HU (Head Unit) has speaker out for rear or not.
If proton cheap out, may not have. Go ask at the persona forums.

Anyhow, you will need amp to drive the Focals. Focals are notorious for needing large amounts of power to sound their best.
Else choose a cheaper/ more sensitive speaker, like Morel or hertz., then you can skip the amp for now.
You not changing your HU anyway, so SQ improvement is marginal at best, and dropping in a focal may not be a good choice.

2. Is the subwoofer enough to reproduce good low end?
Any subwoofer is better than nothing.
we can recommend the sky, but what you get depends on your budget.

3. The sub has on board amp, do I still need a monoblock?
Nope.

4. Will any of the needed process void my warranty?
Maybe, maybe not. You have to Ask your SA.

6. Do you know any accessories shop with good installation  practice?
KF Audio at Aman Suria, or EA Autoworks at Bukit Jalil

7. Anything else I need to know?
As they say, "Crap in, Crap out". Please do not upgrade to just use bluetooth to play mp3.
Use "wired 3.5mm" or "USB" and FLAC files.

Normal Upgrade path :
Head Unit > front speakers > front amp > front sound proofing > subwoofer > whole car soundproof.
Any other path will produce sub-par results, period.

Thanks for the comment. Zero5177 gave some good cost saving measures. So now I’m considering moving the stock speakers to the back, and using the extra budget to change my HU. So now my setup looks like:

-Kenwood DDX6016BT
-50w x 4 output

-Hertz DCX165.3 (Front)
-Coaxial
-peak 120w
-Continuous 60W

Stock speakers (Back)

I’m guessing the 60W is a bit on the higher side, so I’m going to need a 2 channel amp.

Leon 2 channel, class AB amplifier.