setting and using a shell variable inside "sudo -i bash -c" [duplicate]

I have a little maintenance thing to do as super user, from time to time. I would like that to be a one-liner, for easy copying from my self-documentation to a terminal. I reduced it to the bare essentials of where it goes wrong; it involves setting and using a shell variable. See for yourselves:

$ sudo bash -c 'var=VAL; echo "$USER $UID $HOME ** $var **"; declare -p var;'
root 0 /home/myuser ** VAL **
declare -- var="VAL"

You'll of course notice that I didn't give sudo it's -i option, and therefor the HOME variable was inapropriate. So I repeated the command with sudo -i:

$ sudo -i bash -c 'var=VAL; echo "$USER $UID $HOME ** $var **"; declare -p var;'
root 0 /root **  **
declare -- var="VAL"

And there it is: although declare declares that var does have the value VAL assigned to it, echo does not echo it. How can that be?

And by the way, I tried the same command, but starting with declare -p var; ..., to make sure var did not double with some read-only environment or shell special variable; it gives bash: line 0: declare: var: not found, as expected.

For completeness I did the same in a multiliner: opening a proper shell prompt and typing several command lines:

$ sudo -i bash
[sudo] password for myuser:
# var=VAL
# echo "$USER $UID $HOME ** $var **"
root 0 /root ** VAL **

This works as it should be, except that I have to copy and paste line by line.

Can anyone explane why, in the one-liner with sudo -i bash -c, a variable can be set, and is declared to be set, but expands to nothing?

$ sudo --version
Sudo version 1.8.21p2
Sudoers policy plugin version 1.8.21p2
Sudoers file grammar version 46
Sudoers I/O plugin version 1.8.21p2
$ bash --version
GNU bash, version 4.4.20(1)-release (x86_64-pc-linux-gnu)

But here is what @terdon asked, and I never tinkered with that:

$ sudo grep '^[^#].' /etc/sudoers
[sudo] password for myuser: 
Defaults    env_reset
Defaults    mail_badpass
Defaults    secure_path="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/snap/bin"
root    ALL=(ALL:ALL) ALL
%admin ALL=(ALL) ALL

A similar question here on StackExchange explains the unexpected behaviour, as was the question: sudo -i starts an extra shell first, in comparison with plain sudo, and it is that extra shell that first interprets the bash -c 'command ...' as a string, encountering the same problems you will have when you try to insert a plain string containing $'s and`'s into a command.

So I took the super user thing a step further, and added su to the command to make it do what I want it to do; mostly you will want to use su --version, but the su-version in this case works fine too:

$ sudo su - root -c 'var=VAL; echo "$USER $UID $HOME ** $var **"; declare -p var;'
root 0 /root ** VAL **
declare -- var="VAL"

$ sudo su root -c 'var=VAL; echo "$USER $UID $HOME ** $var **"; declare -p var;'
root 0 /root ** VAL **
declare -- var="VAL"