Because of the hack you had to use to get v1.1.21 to work with a newer version of PHP, perhaps you could un-hack it, and use PHP 5.3 which is what this version of SMF was built for, unless you're using another PHP script that has to run on a newer version of PHP.
If you're using headless Ubuntu server, there are ways to install and use different versions of PHP, I wrote a bash script that allows me to change Apache-PHP versions as well as the PHP CLI versions, from PHP versions 5.6 to 7.4 inclusive, using a menu driven script, so that if a forum or CMS script complains that it needs a newer version of PHP, then I just run the script, select a new version, then re-run the forum/CMS install script and check it again, and so on, until I get the right PHP version.
Currently I have installed on my Ubuntu server, PHP versions:-
5.6
7.0
7.1
7.2
7.3
7.4
And I can change to any one of them using the bash script.
If you want to have a look at the script I can post it on pastebin and post a link here.