all but purpose of any upgrade stated.?  why?

id get that HDD as 2nd drive an boot from SSD first , why wait ?, why last booting HDD on earth (LOL)

what don't you like now  , on this PC, sure SDD , but what else?  name one application program that does not run to your satisfactions. PCs can do 100,000 jobs,  can you name one that is not up to par?

is this gamer dreams , be nice to know one app, you don't like? AAA games $60 each ?

the PSU is missing your list.   and is first thing, on GPU upgrades after FITS. (fits and does not hit)

just cut my lid up10mins ago to fit a R9-380 (msi) to my nice T3610,  with custom DIY bumper I designed.

a 200 watt card and runs on all Dell MT PCs here.(but not yours lacks AUX power to GPU.)

I could put any if  25 GPU cards In my LAB CACHE locker and all will run. every one.

the MT is MOBO dell limited for slot power,  it's  75watt slot x16 so that is off the panic grid.

I have 2x (1 at a time) 200watt GPUs that run in my 790 and has so called slot limits, (1/2 true that)

Power is next ( yours)

1650 time. uses a whopping 75watts max full tilt , even with furmark .

dell has no parts lists so I google it or ebay this,  290watt PSU, is par. (and is 8 pin non ATX spec but there is cure for that, if ATX PSU fits.) but 290w should be ok,  for sure with SSD and freeing up 25 more watts from HDD.

so why can this not work. ?

buy  it at Newegg and exchanges are easy.

Not likely working with 9020 MT with 290w power supply.

 1650 is newer UEFI class 3 card.

Secure boot must be off for non dell cards.

GT1030 works fine without any mods.

If you want to use better a GTX 1060 SHORT card works with the optional 365W power supply.

Dell OEM 365w PSU =
7VK45 365w, Huntkey
T1M43 365w, Delta

DVP9W Nvidia GeForce GTX 1050 Ti, PCIe, 4GB, DVI-D/DP 1.3/1.4/HDMI 2.0, 75w
36V90 Nvidia GeForce GTX 1060, PCIe, 3GB, DVI-D/3DP/HDMI, 120w
24K8H Nvidia GeForce GTX 1050, PCIe, 2GB, DVI-D/HDMI 2.0/DP 1.3

Non dell cards will require SECURE BOOT OFF and Legacy option roms ON.

Only short single fan cards physically fit into the case.

Any power supply other than the optional 365W requires MODDIY Adapter.

https://www.moddiy.com/products/Dell-OptiPlex-9020-PSU-Main-Power-24%252dPin-to-8%252dPin-Adapter-Cable-%2830cm%29.html

OPTIPLEX 7VK45 T1M43 365W

This power supply is 8 pin 7VK45 T1M43 365W

I know this an older post but what is the max length of a card that I can fit in a dell 9020 mini tower without removing the hard drive cage? Thanks in advance.

8.1 inches

9 to 12 inches wont physically go into the case unless you cut out the hard drive bays with tin snips.

What is the length limit with the cage removed?? Just curious.
P.S what is getting in the way when the gpu card is too wide because I noticed you said there is a 115mm limit in width?