Your suggested updates appear to be mainly for the 3rd party vendor libraries, such as twig, etc, used by OpenCart. I assume that these libraries already have been updated in their respective 3rd party sources, and we should use composer for an appropriate update in one go, rather than manually patching up these files.

Therefore the opencart/composer.json ( https://github.com/opencart/opencart/blob/3.0.x.x/composer.json ) should be updated to support PHP 8.4. We might loose support of PHP 7.3 or 7.4, but this is a mall sacrifice to make.

Use PHP 8.0, run composer for this opencart/composer.json, and hopefully it will support all of the PHP 8.x versions, including 8.4.

Looks like Twig need updating to 3.9 minimum for PHP 8.4 support. Twig 3.12 drops support for PHP 7. Twig 3.20 drops support for PHP 8.0.

Looks like Twig need updating to 3.9 minimum for PHP 8.4 support. Twig 3.12 drops support for PHP 7. Twig 3.20 drops support for PHP 8.0.

It currently uses Twig 3.8.0, so it makes sense to upgrade that one 3.9, via composer, if that's what's needed. When I have some time I'll look into it.

Everything you think to make it php 8.4.x compatible is okay.
Losing support for php 7.x is the smaller issue on it - or do you want to keep support for php 7.x?