Consolidated available data about C9:
Main differences between C8 and C9:HDMI 2.1 - four full 48Gbps ports
4k@120Hz (2160p@120Hz supported only after the mid-2019 firmware update)
1440p resolution support (gamers only or for people who use tv as a monitor)
VRR with 40Hz-120Hz range (gamers only)
eARC adds support for bitstreaming HD audio (Dolby TrueHD, TrueHD+Atmos, DTS-HD HR, DTS-HD MA & DTS:X
HDCP 2.3 supported only after the mid-2019 firmware update
ALLM (Auto Low Latency Mode) (gamers only)
Variable Analog Audio Out (3.5mm headphone jack)
expanded calibration options
improved temporary image retention
screen is less darkened for news channels that have icons, banners, etc. (less aggressive ABL)
significantly improved input lag for many resolutions/refresh rates (under 7ms at 120Hz, gamers only)
metal stand instead of plastic, larger footprint, more stable
improved WebOS (version 4.5, runs smoother, plus a few more options)
Amazon Alexa support (only after May 2019 03.60.02 firmware update)
Apple AirPlay 2 and Apple HomeKit support (only after the mid-2019 firmware update)
Connect/control various Internet of Things devices
possibly better at avoiding burn-in, time will tell (less relevant for non-gamers and non-news watchers)
Known problems for C9eARC - does not pass-through 6/8 channel LPCM, only 2.0 channel LPCM - the C9 can be forced to accept 8ch LPCM with an EDID adapter - LG has been informed, waiting for a resolution;
eARC - the 2017 eARC compatible models of Denon+Marantz receivers are not negotiating more than 2.0 LPCM (Stereo), no HD bistreaming, Denon AVR-X3400H, X4400H, X6400H, Marantz SR7012, the C9 can be forced with an EDID adapter to accept more formats - this is NOT the LG's fault, but Denon/Marantz fault;
eARC - eARC is disabled with SimpLink/CEC=OFF on the TV | ?bug or "feature"?;
Wi-Fi - the C9 will lose the wireless connection and you have to reconnect daily (wired is stable) - ?induced by a particular firmware or condition?;
Audio - random return to internal speakers instead of the previously configured ARC/eARC | triggers: on Power ON, on source switch | affected firmware: 03.60.04;
Internal apps - (example: Netflix) do not pass-through HD audio via eARC, but lossy formats such as DD+Atmos work (This is also a known problem with the eARC Sony TVs.), must use an HDMI source for HD Audio;
USB - WebOS media player does not pass-through HD audio via eARC, but lossy formats such as DD+Atmos work, must use an HDMI source for HD Audio;
DLNA - unable to pass-through HD audio via eARC, must use an HDMI source for HD Audio;
Updated list for known AVRs vs C9 eARCAVRs known to work with C9 eARC- Pioneer VSX-LX503 - 2018 - requires CEC=ON on the receiver for eARC to work)
- Denon AVR-X3500H - 2018
- Denon AVR-4500H - 2018 - eARC works (ignoring the TV Audio setup) with CEC=OFF and ARC=OFF on Denon and eARC=ON on C9 (also works about the same with the 2018 Sony A9F/AF9 OLED eARC TV, with CEC=OFF on both and ARC=OFF on Denon)
- Denon AVR-X6500H - 2018
- Yamaha RX-V685 - 2018 - (requires CEC=ON on the receiver for eARC to work)
- Denon AVR-S750H - 2019 - (requires SimpLink=ON on the C9 and HDMI Control=ON on the S750H for eARC to work, but with the TV Audio setup?)
- Denon AVR-X1600H - 2019 - (also works with 2019 Sony X950G eARC TV)
AVRs not working with C9 eARC- Denon AVR-X3400H - 2017
- Denon AVR-X4400H - 2017 - (on TV Audio is capable of only Stereo with the 2018 Sony A9F/AF9 OLED eARC TVs internal apps, HD audio does pass-through!);
- Denon AVR-X6400H - 2017 - (on TV Audio is capable of only Stereo with the C9);
- Marantz SR7012 - 2017 - (also no HD audio with the 2018 Sony A9F/AF9 OLED eARC TVs internal apps/Plex)
Source: AVSForum C9-E9 Owners Thread
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To answer your question, solely depends how early you are going to adopt HDMI 2.1 devices. PS5 and Xbox Scarlett is most probably due late 2020, by then for sure - there will be a C10, which we may or may not get the promised top emission OLEDs which was supposed to come this year lol.
If you aren't in a rush to get an OLED now, might as well just wait a few more months. As has been historically proven, LG reduces their OLED TVs by the end of the year. It has happened for the last 3 years and I don't see a reason why they won't follow the same trend.
There are still quite a few C8 left in stock all over the country, which is why you see so many "clearance" sales for them - which every single retailer will tell you "limited CNY or Raya sales only". Lol yeah, CNY and Raya lasted 4 months alright.

Many thank for your insight. Sounds like no need to spend the money now.