Hi Sam,  let me share a couple of thoughts with you.

1. PHP versions change continually,  when they reach a EOL (end of life).  You need to be using the  support versions to keep things more secure and working.
You can check EOL  for php  here:  https://www.php.net/supported-versions.php
You can see even 7.4  if out of active updates and moved into security only.

2.  You are now understanding how customizing your cart has a downside  are not keeping up with changes to various systems.  Perhaps your customization is able to work  with  no or little changes.   Perhaps you should look into make your coding changed into an extension??  You need to rethink how your custom coding is being applied to the core,  and make some adjustments to your process -  as upgrades to php,  OS,  etc is ongoing.

3.  If you have access to cPanel and softaculous,   you could clone your existing site,  upgrade your php etc and see if it works with your custom coding etc. 

We have posted Softaculous how-tos on our info hub:

https://why2central.net/kb/using-softaculous/

Please provide clarification on extending support:

MarketPlace says:
The purchased extension is provided with 120-day support from an author with free upgrades within this time frame. If your order support has expired, you will not get any updates for your extension and developers might refuse with the support requests.
You can extend support time and updates with the purchase of an additional 1-year support term.

If the store is connected to the extension, and the Extend Support button is clicked  -  is the renewal fee ONLY for the additional support? or does one have to repurchase the extension and the extended support?

It has been my experience with trying to buy extended support  that I have to rebuy the extension also. 

thank you for clarify what is suppose to happen.

Please refer to the screen shot   -  I set up the product within the testing site I provided.

this is what that option does ,  it allows admins to control what is seen on the product display page.
This why we made the extensions at our clients request - so  option prices could be clearly shown.

Either of the extension  Option lite or Option Pro allows for the management.

One more thought -  since having an odd ending price is not your desire  you could perhaps use options.
The important thing to know about options is to use a price modifier for the ending price.  This means the base product price PLUS the modifier equals the ending pricing to customer

I made a rough screen shot for you, showing one of the option methods (checkbox)

We have several extensions  that we offer for handling options so the ending pricing is not so confusing.   Below is the links to our manuals so you can see how to provision, etc.   

https://why2central.net/v1/option-lite/
https://why2central.net/v1/option-pro/

Additionally I asked you in a different post what you were selling - something with weight or length or?
We have some additional extensions that handle those types of calculations.

Give the options a try and see if they will work out for you.

what type of products are you selling that you need a fractional amount?

length,  weight,  ??

Did you read this documentation:

https://abantecart.atlassian.net/wiki/spaces/AD/pages/10747906/Quantity+discounts

send us a screen shot of your settings and what is wrong with the pricing displayed and we will try to assist you better

We have found this issue on several of our client sites,   the fix for us has been to configure the email sending using SMTP rather than the system send mail. 

check this AbanteCart doc for info on enabling SMTP mail settings.
https://abantecart.atlassian.net/wiki/spaces/AD/pages/7372837/Mail+Settings