iMac Retina 5K 27-inch 20

Since you mentioned slow boot up times, I concentrated on the performance section of your EtreCheck reports. Specifically the File system, Write speed, & Read speed values.

Performance:

System Load: 1.49 (1 min ago) 1.06 (5 min ago) 0.50 (15 min ago)

Nominal I/O usage: 6.09 MB/s

File system: 19.64 seconds

Write speed: 692 MB/s

Read speed: 2585 MB/s

Those numbers on their own don’t immediately scream “failing Fusion Drive,” but they *do* hint at the usual Fusion-Drive-related bottlenecks you see on a 2019 iMac. The SSD portion of a Fusion Drive generally delivers the high read/write speeds you’re seeing here—your 740 MB/s write and 2.6 GB/s read are perfectly healthy for the NVMe tier. The issue is that the HDD portion (as you have suspected) can still cause ugly delays, especially during boot, when macOS may be pulling data from spinning media rather than the SSD cache.

That “File system: 19.64 seconds” line is the most telling part of the EtreCheck output. Normally that value should be down in the low single digits. When it creeps into double-digit seconds, it usually indicates very slow responses from the mechanical part of the Fusion Drive, excessive fragmentation, or the OS waiting on the HDD tier to spin up and serve system files. None of those mean “imminent hardware failure,” but they do match the classic slow-boot complaints.

What I’d recommend are the following options:

• The lowest-effort option is backing up and doing a full erase/reinstall to defragment the HDD and refresh the Fusion layout—this can help, but it rarely solves slow boots for long. (I highly recommend this one BEFORE the next two.)
• The next option, that actually delivers a noticeable performance jump, is migrating macOS to an external SSD or splitting the internal Fusion Drive and using the internal SSD alone.
• Finally, you have the option to completely replace the Fusion Drive with a single SSD.

One thing to take into consideration, is that although a 2019 Mac is currently considered "Supported" by Apple, it is quickly approaching the "Vintage" category, and that means it will get increasing more difficult to get repairs from them. It can still support the very latest version of macOS (Tahoe), but that will be the last version that will still run on an Intel-based Mac ... so, this is something you may want to consider before putting too much money into "fixing" it.

Ref: Obtaining service for your Apple product after an expired warranty - Apple Support

Tesserax wrote:

One thing to take into consideration, is that although a 2019 Mac is currently considered "Supported" by Apple, it is quickly approaching the "Vintage" category, and that means it will get increasing more difficult to get repairs from them. It can still support the very latest version of macOS (Tahoe)

Sorry. Sequoia is the end of the line for the 27" 2019 iMac. Among Intel-based iMacs, only the 27" 2020 model received Tahoe. (The 2019 Mac Pro desktop minitowers and rack mount computers can run Tahoe, but that doesn't help the 2019 iMacs.)

If the internal Fusion Drive is not actively failing and throwing errors that crash or slow down the whole machine, the OP's best course of action may be to add an external SSD and to start up from it. That would avoid the cost involved with paying someone to do surgery on the iMac, or the risk involved in doing the surgery at home.

To start, the 2019 27" 5K Retina iMac had the following storage options:

• Fusion Drive in 1, 2, or 3 TB capacities.
• SSD Drive in 256 GB , 512 GB, 1 or 2 TB capacities.

Is the only issue a slow startup? What gives you the impression that the Mac's drive(s) is(are) the culprit? For example, did you run either or both an EtreCheck or a DriveDX report to confirm this?

Again, a "2TB drive" can either be a combo (HDD/SSD, aka, Fusion Drive), or a singular SSD. If you don't want to post your current EtreCheck report, that's up to you ... but, without it, there is little we can go on other than what you tell us.

Ah, you are right. Thanks for catching that, and the correction. Much appreciated.