essentially had to replace the full body of phone, major issues

Heavily damaged phones can have multiple issues and sometimes they are just not cost effective to repair. That said, you will need to strip this phone down to bare necessities and test every flex or component until one of them causes the boot-looping issues.

Start by disconnecting the battery first (always!). Then disconnect the digitizer, LCD, front camera, rear camera and home button extension flex (see this guide). Now re-connect the battery; you should only have the battery and Lightning Dock flex connected. Connect a known-good, preferably Apple-original Lightning cable to an iTunes enabled computer.

 Does iTunes recognize your phone? If so, then leave it on for a while to see if it is charging the battery and not looping. You can also use 3uTools which is a better iTunes-like” tool that will show you the status of the phone as long as it is connected (and booted). The re-connect one flex at a time until the phone stops working(remember the battery!). I would start with the rear camera, LCD & Digitizer then from camera flex.

If the bare-bones setup does not get recognized by iTunes or loops, then you most likely have a Lightning Dock Flex or logic board issue. You can try a replacement Dock flex and if that doesn’t help, then send it (or find a local) to a repair shop that specializes in micro-soldering repair. Sometimes the fix can be straightforward (i.e.  a blown filter) and relatively economical.