Then one day a fortnight ago, I chanced upon another solution. Some people are speculating that because the WiFi signals is clashing with Bluetooth signals as they are both in 2.4 GHz, its affecting the WiFi. I was using Apple Wireless Keyboard & Touchpad. I have friend who had similar setup who seems to have the same issue.
So the next day, I spent 464 USD just to upgrade my router, get a wired Keyboard and a wired Mouse.
I have definitely splurged on these hardwares but at this point I am so frustrated and desperate that I need to make sure any hardware issue is out of the question. These are the only variables I can control, anything else felt hopeless.
I then jumped on 5 GHz WiFi and stop using bluetooth. My WiFi situation have improved a lot since then. Not perfect, but not un-usable.
The point is that I could have saved those money. To spend so as to fix a bug felt so Window'ish. It use to just work, and glitch are bearable, now I always worry of upgrading.
Anyway, those money could have went into buying Apple stuff.
I also read somewhere it was related to Handover protocol using the same bandwidth as wifi. This sounds more plausible. Then having bluetooth + regular wifi + Handover wifi is too much and the connexion suffers. I which case there's not much Apple can do unfortunately, except rethink the whole stuff.
I'd say in any case the future in that matter doesn't look bright.