So I left libllvm9 installed, and proceeded with the upgrade. The ONLY error I saw, which did not stop the process, was a "Network Unreachable" complaint when trying to install some mscore font. I noticed the libllvm9 package was removed during the Autoclean phase, near the end. I saw that libreoffice core was deconfigured (broken by libre office draw).
When I rebooted into LM20, I saw the graphic page with the purple background and the dots. An error appeared there briefly, something about uvcvideo failed to query getinfo. Then that page disappeared and I was in console mode. I was able to log in, but could not start X because there was no X installed. I tried to log on here with Lynx but was blocked from posting because that's the preferred browser of bots. So I reinstalled my previous snapshot of LM19.3 using timeshift.
I believe I followed the instructions correctly for the upgrade. I had the thought to follow all the steps again up to mintupgrade and then deliberately reboot into console mode in order to run the final "mintupgrade upgrade" command, but this is not what you recommended to do, and I'm concerned that would crash.
I just now got a system notification that a new version of LM is available, and click the button to upgrade. Should I just click it?
Not knowing what other information to provide, here is the output of inxi -b. Maybe the relative age of my PC has something to do with this.
System: Host: Achernar Kernel: 5.3.0-62-generic x86_64 bits: 64 Desktop: MATE 1.22.2 Distro: Linux Mint 19.3 Tricia
Machine: Type: Desktop System: Acer product: Aspire X3910 v: N/A serial: <root required>
Mobo: Acer model: WG43M serial: <root required> BIOS: AMI v: P01-B2 date: 03/24/2010
CPU: Dual Core: Pentium E5800 type: MCP speed: 1922 MHz min/max: 1203/3203 MHz
Graphics: Device-1: Intel 4 Series Integrated Graphics driver: i915 v: kernel
Display: x11 server: X.Org 1.19.6 driver: modesetting unloaded: fbdev,vesa resolution: 1920x1080~60Hz
OpenGL: renderer: Mesa DRI Intel G45/G43 v: 2.1 Mesa 19.2.8
Network: Device-1: Intel 82567V-2 Gigabit Network driver: e1000e
Drives: Local Storage: total: 931.51 GiB used: 362.12 GiB (38.9%)
Info: Processes: 192 Uptime: 3h 15m Memory: 3.81 GiB used: 1.70 GiB (44.6%) Shell: bash inxi: 3.0.32