I couldn’t do it, but then I also don’t have my own iPad either (although there are plenty in the house).
The iPad is great for consumption of stuff, video, facebook, twitter etc etc, with a bluetooth keyboard it even makes a great email tool and (basic) document creation. Models with Pencil support can even be used for more creative stuff…
But as has been said, file support is lacking, storing all your photo’s and videos is 100% reliant on online services same for documents. Backup of those things is hard (impossible?)… An iCloud Storage/Office 365/Google drive subscription would be recommended.
I still feel like using an iPad as a single computer, while possible, is just a world of compromise and pain to force it to work. A real computer is still so so much more flexible.
What model is the current iMac?
Assuming it still works without hardware issues and it has USB3.0 (late 2012 onwards), I would do is buy one of these:
Install OSX onto that and boot from there instead of the internal drive, which is what we do with our 2017 27" iMac. Our is then velcroed to the back of the iMac with 3M velcro tape. You can then format the internal drive and use that for extra storage if you need it.
If you’re running a 2012+ machine you already have 8GB RAM which is adequate, upping that to 16GB would likely not give you any real benefit from the sounds of your requirements.
If you were really keen you could pay someone to pull it all apart and install an SSD inside, but this is likely to significantly up the cost for little gain…
So really, for ~$150 you get a pretty quick machine. Buying a new iPad as well gives you the best of both worlds which is what I would recommend.