You can't pass-through A/V signals on RPi, but you could trigger an HDMI relay when the RPi turns on.
If your RPi is always on, you could write a script to trigger the relay by RC.
PS: I know that so called "sharing switches" exist for USB. They automatically switch to the active source. Maybe switches like this exist for HDMI, too.
Router - ZyXEL Keenetic Ultra II, no 2.4, only 5GHz...
I remember that other RPi 4B users have the same problem with 5GHz WiFi. Because it works with the same Hardware (same RPi, same PSU) on Raspbian Buster, I assume it's a bug on LE. My guess about the bug is that LE uses a different energy management. 5GHz needs more energy than 2.4GHz. Maybe write a bug report to give the bug more attention. I think that's all you can do.
Just a little explanation: On a Linux OS like LE, the OS selects the the eGPU (your Nvidia) as default. It seems like the driver for your eGPU is not available, so the OS switches to the iGPU (your Intel). If you really think it's worth to go into depth, provide a log file.
If you are running the same add-ons on LE and CE, then one of those could be the problem.
Your benefit now is to have one device for testing, and the other for "production". 
Run a vanilla installation. If you don't have issues for a couple of days, add one add-on, and test for a couple of days again. Do this for each add-on. Hopefully you will find the add-on, which crashes the OS.
Is there a way to just add this functionality to the default skin?
Sounds like a feature request. 
PS:
It's important to understand that power-off on RPi doesn't lead to a stand-by mode, as you know it from other devices.
The RPi can wake up other devices, but not vice versa.
USB and HDMI ports will not respond to any signals after power-off.
Your only option to power-on while connected to power supply is to use the GPIO.
vtsteez243 offered the best solution, go for it.
You still haven't understood what we are writing here.
What you want is technically impossible.
HDMI-CEC can't turn your device on.
The specs say that you have 1080p, so you were not correct at post #3.
That's a game changer. It means, not all available video modes are detected by LE.
Now my suggestion is to check your HDMI cable. Try a different cable or buy a new one.
You need an HDMI 2.0 or 2.1 standard cable be future-proofed for 4K.
Don't be sure you really used 1080i on OpenELEC, because your TV has 1080p.
It is correct, but it doesn't help. Your TV is "HD ready" standard.
According to this it's very difficult to deal with interlaced video modes. So I think Kodi has good reasons not to support interlaced modes.
Maybe you had real interlaced modes on other RPi OS'es, but on LE (and other multimedia OS'es) your chance is very low to make it work.
Why do you need ad blocking on LE? I don't see any ads at my LE.
Yes, you're probably right, HDMI disconnection shouldn't do any bad.
I have noticed that the crash happens nearly every day around 5am.
That's interesting. To my knowledge, Kodi doesn't have a stand-by mode (as well as RPi itself), but CRON jobs will run occasionally. I just had a look at my /etc/cron.d folder. The only service I found is e2scrub_all, which checks all EXT file systems. Maybe that check runs daily, and crashes Kodi in case of errors. So you could check all your (network-) connected EXT devices for errors manually.
Thanks for testing. My math was that half of the progressive rate would be interpreted as interlaced rate. Unfortunately that doesn't work.
I'm running out of ideas now. As chewitt mentioned, you maybe never used an interlaced mode on your other RPi.
Remove the line hdmi_ignore_cec=1 from config.txt.
Then faking an interlaced mode might work (1080p at 30Hz):
hdmi_group=1
hdmi_mode=34I there a way to use tvservice to show the exact mode currently used?
tvservice -s
Then you probably have an older TV, right? User donbrew made an older TV work with this config.txt. Give it a try.
Looks like a problem of interlaced video modes. Has your TV the ability to display 1080p?
OK. Because we haven't found any cause in the LE log yet, I guess your receiver stops HDMI connection, and LE can't deal with it.
The reason for this could be too much HDMI data coming from LE. If you can change the cabling to avoid the receiver, give it a try.
