Update: Finally, I was able to squeeze some time to get this issue fixed by creating a case at MyHPSupport, since my ZBx2 is still covered by “HP extended warranty” (or “HP Care Pack”), which includes “N-CSR Battery Replace”. They actually provided onsite battery replacement, and an engineer from Hemmersbach UK (a contractor providing service on behalf of HP Support) came to my home to replace it, and it took about 30 minutes in total, and so I don’t need to send it back, which is good. Overall, It took one week from case creation online at MyHPSupport to the engineer’s visit for onsite battery replacement, so the time frame is pretty good I think.
According to http://h10032.www1.hp.com/ctg/Manual/c06111498.pdf#page=49, replacing the battery is relatively trivial after removing the display panel assembly, which requires Torx4 screw driver and flat-bladed plastic tool (plus very careful operations, patience, and time). However, the engineer replacing my battery told me that he actually used Torx5 screw driver instead of Torx4, so that Maintenance and Service Guide PDF is inaccurate or outdated, although it’s still listed in the “Manuals” section of https://support.hp.com/us-en/product/hp-zbook-x2-g4-detachable-workstation/13556323.
The HP part number of the replacement battery is 856843-855, and it seems to cost £80~£100, but it’s hard to find; 3rd-party manufactured compatible battery costs only ~£50 at https://www.amazon.co.uk/dp/B07SSY6BG8/, but the quality could be an issue.
The battery issue seems to be related to HP Business Notebooks and Mobile Workstations - Battery Not Charging | HP® Customer Support, while I have always set the “HP Battery Health Manager” to “Maximize Battery Health” (limiting the max charging state to 80%), and after extremely long time of using with AC power plugged in, the battery stopped charging when it should be charging, and gradually discharged/lost its capacity to 0%. And it was too late to update the BIOS to the latest version, as the battery was already dead.
Now after more than two and a half years, I was finally able to use my HPZBx2 properly without hardware or driver issues, although now I actually and merely use it at a Cintiq replacement via Windows wireless projection (Project to this PC; see this thread List of Software that turn tablet into drawing screen), and I can’t be more happy to finally get rid of my 8-year old ThinkVision LT1423p, which has only 1600x900 resolution and is not suitable to be used as my main display (in order to enable single finger panning in OneNote). The Windows wireless projection will create a virtual wireless adapter called “Microsoft Wi-Fi Direct Virtual Adapter” and there will be a “Miracast display device” shown in devmgmt.msc. Depending on your graphic card’s power, number of displays/monitors connected, and your Wi-Fi adapter’s speed, you might get different resolution and refresh rate on the wireless “Miracast display device” via “Microsoft Wi-Fi Direct Virtual Adapter”. For my case, if I get ThinkVision LT1423p connected via USB 3.0 with DisplayLink driver, I can only get 2560x1440@30Hz on HPZBx2 as a wirelessly projected display, but if I disconnect the ThinkVision LT1423p (and if the Intel Wireless-AC 7260 on my Host PC ThinkPad W540 is working properly at 5GHz), I can get full 4K resolution at 50Hz on the wirelessly projected HPZBx2. Even though I could still feel the lag when hovering or taking notes in OneNote, it’s light years better than the wireless version of ThinkVision LT1423p. And hopefully I will be able to find some time to finish the Windows deployment and data migration to my HPZBx2 during the year-end holiday, so that I can free up my 8-year old ThinkPad W540 and use it as a data server.