Is it worth upgrading these components?

When it comes to the CPU, it can help but the effort to performance gain ratio isn’t as great as you think. It’s not something I’d personally do, especially with the Mac being so old further limiting any gains. It’s often a completely different question in a system with easy access to the CPU, but you especially need to be at the top of your game to not screw the CPU swap up on an iMac. Remember, this isn’t like my custom desktop PC where if I got a good deal on a 9th gen i7 without the IGP, I can swap it in 10 minutes so the effort to performance gains are easier to justify, even if I’m coming from a 9400f and it isn’t much better and I need a Hyper 212 EVO as my Intel stock cooler will not cut it. This is because I’ve done it, my install risk is basically zero and if it doesn’t help much it wasn’t hard to swap out, so there’s less “upgrade regret”. The “professional rate” to get this done will total it out, so the CPU on an iMac is firmly a DIY job once it’s a few years old (unless you know someone who will take it on at a rate that isn’t machine totaling).

As far as SSDs go, you need a SATA II compatible SSD due to the chipset in these - if you had a 2011/12 at the bare minimum you wouldn’t need to worry about this! Yes, the RAM will be a upgrade worth going for.

The most I would upgrade on a machine like this is the RAM and installing an SSD - the CPU gains vs risk of destroying the machine are not worth it unless you get the CPU super cheap (due to the extreme PITA factor on an iMac, 10-20%, with a 10% buffer at most. If it fails here, it’s not worth pursuing).