TLDR.. I can confirm for you that it's not a Kodi/Quasar/Neo U9-H issue.. Before you throw in the towel, need to first figure out if you've done what needs to be done and your home network is in excellent shape..Wrong place to discuss such.. So do bear with the answers I'll give..

1... I hate pre-installed stuff. I know it may seem like the seller is helping. But if the seller just tosses all sorts of crap onto the box. They do more damage than good. Minix makes one of the better firmware out there. Clean and neat.. Use that advantage... So I say, nuke the box, check for updated fw.. Update if any and nuke it again afterwards, just in case to ensure its clean.. Install the latest Kodi on it.. Grab from Playstore. This is the Kodi dedicated thread, can't discuss anything else.

2. Again.. Kodi dedicated thread..

3. An expected response.. Can't fit all 12 litres of water into an 8 liter bottle... Use external memory.. I.e., a good fast usb or normal spinning disk external hard drive... It'll fit then.

4. Need to know more.. If the file isn't properly seeded, it'll lag cuz Kodi can't play a stream it doesn't have, now can it.. So has to wait till it's in and then continue.

5. This might be behind the issue of #4 above... However, I know that Minix boxes are arguably one of the better boxes where Wi-Fi connection are concerned. So if you are still experiencing Wi-Fi issues.. Best to get a better router or and access point connected via cable and installed downstairs.. Personally, I like everything cabled and dislike powerline adapters due to their limitations (you can read up about this). I stay in a typical same floor apartment and still, I drilled a hole to run 2 separate 15m cables to another room. So that the main PC gets full speed directly from the primary router and the other cable is connected to another router (as an access point) to spread the rest of the connection all over the house. Results? Full speeds where needed and zero blindspots.

You are welcome.. Sure, some people find it messy to drill holes and run cables.. I get that.. So yeah, you can try the wifi extender/powerline route. Usually with such, the internet speed is halved. So if you normally get say 100Mbps off the primary router.. the powerline adapters will be getting around 4xMbps tops. Maybe better if it's better equipment, those are pricier. Alternatively, seriously consider getting a single more powerful router, place that in a central position and hopefully cover both floors. Either options is dependent on the size and interference..

Alright... Need to return back on topic.. Just sort out the home network & external storage issue.. Reset the box and setup.. It'll be good.

Ah.. No surprises there.. I actually forgot or rather I assumed that you'd already tried using the 2.4Ghz network and still got poor results. Nevertheless, you've seen the positives as expected.. Now need to work in that daemon error. If it's the io error.. Just follow the advise given in the previous page or so. Install the modded Quasar zip file over the original. Reboot. That'll fix it right up. Alternatively, since you've got Elementum working superbly, you could just stick with that unless something breaks or it decides to perform badly. But always good to have both options.

RM180, IMO..is a small price to pay for a cabled network, especially since you'll be using the device on an almost daily  basis. You'd want full stable speeds. Ask TM, does that price include the cables and what type? If it does, ask for a price without the cables.. So you can get the cables yourself. In my case, I was able to get my own cables and only paid a lower price (under RM100) for standard drilling and cable running.. This is a price fixed by TIME themselves for all their installers. Doing this allows you to buy better cables and don't worry about re-running them again in future. Else for mine, they would have given CAT 5e... So since I was getting mine, I got UGREEN's CAT6 cables instead. Better performance. They work great. So I recommend you get them should you go down that route. It'll also help for NAS sharing purposes in future too.

Not really. Unless you want to store the library files on there too. In which case, yes.. You have to create a Library folder with separate  sub folders for tv shows and movies... This in addition to another folder for downloads. exFAT remains the best choice for MicroSD cards. I prefer keeping the library on the faster internal memory.. You can have it set to both internal and external sources..