My Mac HD is not showing in the disk utility

My 27" retina imac 2015 has slowed to a crawl and I suspect the old mechanical Fusion drive is faulty. I've tried clearing down and resets etc. but to no avail. I've no need for the data so would look to upgrade to SSD anyway.

The issue is,

• going thru recovery mode, in disk utility I can only see the macOS Base system on an APFS system volume (circa 1GB)
• however once turned on I can see the macHD no problem (circa 3TB capacity of which Applications use 700GB and IOS another 650GB), albeit slow
• I cannot erase, restore or unmount given there is no image lsited in the disk utility

NB. I can get to the terminal screen.

So if I were to simply replace the HDD with a new SSD would that work and what watch-outs to look for in doing so.

thank you