You should stay on the new Ubuntu and install php7.4, then update from major version to major version.
You should use → the php-updater script ← to install the normaly not shipped php versions.
The Script will add the required apt sources to obtain php 7.4.
After you have done that, you can install exactly the same modules on the new server as you had on the old server.
This will give you a list of those packages on the old server:
dpkg -l|awk '/^[hiu]i\s+[^\s]*php/{print$2}'
Then you can copy all the php.ini files into the new server. The php updater script can migrate those old settings to the new php versions.
Much and good luck,
ernolf