Child Theme + Plugin customized code not lost + back ups + sign up working for option 2

Looking for an experimented dev who knows well how to customize wordpress plugins and knows eventually how to use OVH, to do the following:

- check why one of the back up plugins is uninstalled and cannot be installed -> error
+ why the other installed back up pluggin doesn't want to create back ups
check error and fix both pluggins

- Wordpress plugins have been customized and need to install child theme to reduce the risk of loosing code when we update the template or wordpress

- Three plugins are extremely customized -> need to find a way to place the code somewhere, so when I update the template or wordpress I don't loose the code. Use hooks if needed to modify plugins without editing them.

- Identify issue why a sign up form related to a paid subscription doesn't work -> why it doesn't work and solve it. Workflow:
a- click on option 2
b- once redirected on log in -> if users clicks on sign up instead of log in, then
c- redirect to plan page ... after that the workflow is the one of option 1 -> until payment it's all done already.

Normally workflow of log in is already ok, then the user just logs in and if he has enough credits he will buy or he has the option to buy more

The job will be considered completed if:
1) Back up pluggin 1 and 2 both work -> milestone 1
2) child theme is properly installed -> milestone 2
2) at least two out of the three customization plugin codes are not deleted if theme is updated -> milestone 3
3) the issue with the sign up form that doesn't work is fixed and workflow works. -> milestone 4

Milestone no completed = job not done

Deadline : 3 days max