Is it possible to upgrade the RAM, CPU and install SSD on my iMac 2019?

You actually have my preferred machine iMac 2019 — it will run Mojave and latest macOS sequoia

the problem 1 making it unusable is the fusion configuration — it’s 3percent fast ssd and 97percent slow 5400rpm spinning hdd (those HDDs top out at about 80MBs sequential read/write speeds)

problem 2 would be the 8gb minimum ram

unless you can cut the screen assembly off flip the main board and upgrade the ram and fast (NVMe) ssd — or at least replace the SATA hdd with a SATA ssd (there are utubes on this procedure)

you are stuck

i recommend investing in an ACASIS 40Gbps M.2 NVMe SSD Enclosure, with Cooling Fan, TBU 405 Pro

and a Western Digital 1TB WD Blue SA510 SATA Internal Solid State Drive SSD - SATA III 6 Gb/s, M.2

get the BLACK if you can afford it -- but Samsung (EVO 970 Plus for example) write speed throttles down in external enclosures (in my experience) go with WD on external enclosures

then reinstall your system on the new ssd

or

clone working drive your current over to the external SSD

and Erase the Fusion for storage

that ACASIS Thunderbolt 3 external enclosure will give you 2700 MBs sequential read/write through the iMac thunderbolt3 port

and that ACASIS external will even be used with usb2 machines

one more point

some will say minimum 8gb RAM isn't so much of a problem in those fusion Macs -- I would say never buy a machine with minimum RAM unless you can upgrade it (it's not soldered to the main board, as in the new iMacs, for example) -- you don't have to pay someone $300 labor to completely disassemble the unit to install new RAM DIMMS (as in the 2019 iMac 21.5" 4K line, for example)

why is buying the most RAM you can afford important

when the OS (or Photoshop, for example) runs out of RAM it has to write/read so-called swap/scratch data to the drive -- where it could be storing it in additional RAM (which is lightening fast in comparison)

if that drive is the old style spinning mechanical SATA Hard Disk Drive HDD (at 80MBs speed) -- that's another huge hit on speed and spinning wheel waiting while the machine catches up -- the tiny NVMe SSD (at least 2000 MBs speed) that completes the Apple fusion configuration carries some of the load, granted

but the SATA bay is much better served with a SSD (450 MBs speed) -- and replacing that HDD with a SSD is a much less hairy upgrade than the RAM -- and makes the iMac "usable" again...

So here would be my advice to anyone buying a new M4 Mac today:

buy the extra RAM when you order the machine (you can add fast external storage later, but the RAM is soldered to the main board you will be stuck with it out of the box)

Opening and resealing any of the iMacs from that era is tricky and probably a job best left to a repair shop. (Which will probably run you close to $100/hour just for labor.)

If you get more RAM, I would suggest ordering it – or having the repair shop order it – from Other World Computing.

Other World Computing – OWC Memory Upgrades For 21.5" iMac with Retina 4K Display (2019)

Macs can be picky about RAM, and the other long-time "go to" vendor for Mac RAM (Crucial) doesn't seem to be in that business any more.

Rather than spend money on RAM that will probably NOT make you iMac any faster, instead please post a system config report for our review. We may see the true cause of your "slows." Fortunately there is a safe, secure way to do that. Odds that you have a RAM issue are very low.

As for pros, I've talked to some who refuse to work on glued-together iMacs due to the labor cost to their customers and the high chances of something inside getting damaged. As rkaufmann87 points out, you have to virtually gut the computer to access the well-hidden RAM slots.

If you want a data-driven evaluation in this setting where we can neither see nor touch your computer, please post an EtreCheck report. We can quickly and within the limitations of these forums help you determine what issues are at play without our playing a protracted game of "20 Questions" with you that could go on for days. 

EtreCheck Pro is available here:

https://etrecheck.com/index

The free version will do nicely for this purpose, although the app is worthy of our financial support.

We can see hard data about drive performance, software issues and interferences, and RAM usage. Etrecheck is the development of a long-serving and trusted ASC contributor. It is a reporting app, not a "fix-it” app, expressly for displaying information in these forums to help us help you remotely. It will not reveal any personal or secure information.

To avoid scrambling the output, please see this excellent user tip on how to post long text reports like EtreCheck's into a forum response:

How to use the Add Text Feature When Post… - Apple Community

...I added an external SSD

Etrecheck will even show us quickly if that effort was 100% successful.

NoeValleyJim wrote:

....But the 8GM of RAM is now a major constraint....

If you notice a lot of swap space being used, and if the boot hard drive is filling up and does not have much free space that can cause some slowness. Have you checked your memory usage?

Activity Monitor

You can see how much memory you are actually using, and if you are using Swap (virtual memory space on the hard drive) which can slow things down.

https://everymac.com/systems/apple/imac/specs/imac-core-i3-3.6-21-inch-aluminum-retina-4k-early-2019-specs.html

Yes it’s possible however it would require a 100% tear down of the computer which could be expensive. At this juncture it may be wise to consider upgrading to something like a new M4 Mac Mini rather than investing in an older machine.

Running Activity Monitor, and leaving it open to the Memory tab while you run the the sort of regular workloads that might use a lot of RAM, is a good way to tell if you have enough RAM.

Both the color-coded Memory Pressure graph and the numbers next to it can be useful. On the graph, Green means that the Mac thinks you have enough RAM for what you are doing. Yellow means RAM is marginal. Red means that the amount of RAM is insufficient and probably hurting performance.

I have a 2019 iMac and use an external TB NVME drive and works fine. I would follow what -g has suggested. If it is still very slow then it means you have some software issue (s). I would not tear it apart to make any updates. Too risky on breaking something.