Since this effort is quite involed. Will not outline some of the basic steps that can easily be found on internet. Anyone uncomfortable with this probably shouldn't take on this project
Do this first to get Apple's latest firmware ( boot ROM can boot off APFS ) and SSD ( don't want to be running later macOS on HDD haha )
SSD installation details here
Follow GRML flashing guidelines
I had 1 iMac that couldn't communicate remotely properly so had to use alternative CH341A flash hardware. This hardware doesn't work on all GPU card ROMs. Some (maybe newer ones after 2015+?) have lower height ROMs and cheap flash hardware clip can not make contact to the ROM. See CH341A section here
Many guides on internet. Metal GPU upgrades is dependent on OCLP so learn how to this on a working pre metal GPU upgrade Mac hardware first.
AMD card options can run latest macOS. nVidia cards gradually lost macOS features >= Big Sur
GPU Selection Table Focus on the green color text
Note Apple's Time Machine, Carbon Copy Cloner, Super Duper!, Migration Assistant all have some issues interacting with OCLP. OCLP is patching the system and the traditional tools is knowledgeable about the system so don't work well together.
Best data backup is probably just a full copy of your key user data under the user folder or where ever valuable data is kept.
Applications should jsut reconsider reinstalling.
It is possible to migrate to OCLP and change GPU and perserve everything but its intricate and probably should be well experienced before making such effort.
09-10 iMacs are not too hard. Many guides online and some key tips here
11 iMac (at least the 27") are more difficult. GPU card can not be removed without pulling the whole logic board with all its connectors and form fitting construction.
Make sure SSD is wiped blank (no EFI boot section) so when the iMac boots with not yet flashed vBIOS AMD card and show blank screen, it can at least boot the GRML stick to flash the vBIOS.
Aiming for simplicity, this website suggest smaller GPU cards which should have the simplest installation effort. Few key points are
Some GPU cards will have X bracket with smaller bolt holes. Easiest to remove these bracket (hair dryer or hot air station ~150C) and gently push out the 4 bolt holes to remove the bracket. There are tiny components on the board to be patient and use heat. Adhesive will loosen when heated and stick on tight again when cooler.
Reuse the original GPU's X bracket
Measure top of GPU die against the top of the board for the old and new GPU. Thats the copper shim size you need to use. For these smaller GPU cards, GPUs are < 150mm^2 so the typical 15x15mm copper shim works fine. Cheap copper shims are pressed cut and will have tiny lip. Sand those off with1000 grit sandpaper to avoid forming a gap. All 3 iMacs I've done from 2009 and 2011 required 0.5mm copper shim.
From the earlier step to check if can use GRML or CH341A, you should be all set on how to use this tool to flash the vBIOS
https://sites.google.com/view/imac-metal-gpu/grml-flash-gpu#h.kkb8lb1nn4vx
Consult this table for your GPU card, links to post with vBIOS should be in there. And of course there is Post #1's tables.
This process is quite mature. Many guidelines exist online. Key suggestions and metal GPU upgrade requirements are
Macs Fan Control (MFC) provides sensor reading for the GPU chip and heat sinks
Run Valley graphics benchmark (ignore its GPU temperature display, its likely wrong) for 5-10min
Make sure GPU chip and GPU heat sink temperature are within a few degrees C of each other. For these smaller GPUs, temperature will probably be 60C or below.
Post with more indepth details