LG has released a new firmware to resolve the reported dimming issues for 2018/2019/2020 OLED TVs.
There is a new setting in the IN-START service menu, to the same page where you disable the TCP.
Its called "GSR: Enable".
By disabled that GSR function, from IN-START in SM, it will disable completely any dimming functions of LG.
That new setting will be available to 2018/2019/2020 TVs after the latest FW updates.
Some countries will not get the automatic update notification, so it will require to access your local LG site to download and manually update the TV.
I believe that it's disabling the two functions for fixed pixel detection.
LG's have two detection processing functions:
Fixed image region detection unit + Fixed image determination unit.
These functions will alter the WRGB values (reducing the driving of subpixels and increasing the W subpixel slightly usually), and this will affect the luminance.
The algorithm counts to make that change as not so be noticeable to the observer.
However, many users can detect that side-effect, it can notice the luminance reduction.
When you have TCP: 0, it will reduce some of these functions, but it will not eliminate all dimming functions.
GSR OFF will disable any of such dimming functions altogether.
I think LG released that setting for not letting Panasonic HZ2000 (not the consumer model, but a particular model thru broadcast retailer) to not have that function as exclusive.
The primary purpose of this fix is for people in post-production where they use LGs as clients view SDR, which was a known issue to colorist channels.
There several methods to access SM, the full info here