Hello,
Hope you are doing well!!!
I have read your all project issue and your upgrade worpdress concern. I have already faced this issue at my personal website only. What it is happened, when you upgrade version, it upgrade only worpdress not theme or plugin. In wordpress , many functionality interlink with each one. Now, may be some plugin code is not compatible with your version or vice versa. I can resolve your issue , How much time it is required , it can say after seen of current status of website.
Time and cost depends on after check of website.
Apart from I have more than 6 years experience in IT industry. I have great hands an experience in core php, Laravel, CI, CMS (using theme and customization), Framework, Node JS along with mobile app development and UI/UX. I can manage easily your task.
Please discuss me at once in chat box.
Thanks and Regards
Swati Garg