Those are good iMacs. The CPU is upgradable, although limited to just a few options. The flash storage models are way way better than the fusion models. Although you can put in a SATA SSD.

No. It's not even close to worth the upgrade IMO. You are talking about spending hundreds of dollars to net maybe a couple percent of performance... It's a fun feeling to max out nine year old hardware, but why though? Edit: It dawned on me later.. a base model $429 CAD iPad, is faster than a maxed out 2012 iMac.

I mean.. look at the benchmarks. The 680MX is up to 40% faster in some tasks... but compared to modern hardware, a GTX 1080 is a 500%+ increase compared to the 675MX. A 3080 is 1200% faster! The same honestly goes for the CPU. Depending on how much that i7 sets you back, you'd get more bang for your buck spending any extra cash on more solid state storage.

That HDD is going to be the biggest bottleneck in general. The CPU/GPU difference are so minimal on iMacs from back then, it's kind of a joke. They are thermally constrained to a point where it's just not going to make a huge difference. I mean compared to a base model M1 Mac mini for $899 CAD? That little box would be twice as fast as your iMac even with the i7/680 upgrades. (Obviously that is still more money, and you'd still need a display and who knows what else... but you get what I'm saying.)

So no, I suggest you save your money. Work with what you've got for now. Focus on content for your YouTube channel. As your budget increases for gear/staff, look into newer and faster stuff.

Unless your content is doing this upgrade? Maybe then it's worth it?... maybe.