Would it be okay to upgrade from Big Sur to Sequoia? I have an iMac 2020 (27-inch, 2020. Intel Core i7).

If you are hesitant to upgrade to Sequoia, you could upgrade to Ventura or Sonoma, instead. Adobe has a policy of supporting only "the most recent three", although at the moment, things seem to be more complicated than that. It looks like Photoshop might still run on Monterey and like Camera Raw might require at least Sonoma!

Adobe – Photoshop system requirements

Adobe – Camera Raw system requirements

The current versions of Apple's own Pages, Numbers, and Keynote also require Sonoma or later – so for updates to those Apple applications, you need to have one of "the most recent two", not one of "the most recent three."

By default, the App Store likes to hide old versions of macOS, so to get Ventura or Sonoma, you would use the links in this Apple Support article.

How to download and install macOS - Apple Support

If you do decide to upgrade the system before you do remove any "cleaning", "optimizing", "speed-up", anti-virus or VPN apps on your Mac. They are not needed and have a history of causing performance problems.

You should always be backing up your files!

All storage devices fail and they never fail at a good time. OS upgrades can go sideways at any time. Gremlins can appear at any time and wreak havoc with your computer. The only way to recover the data lost when something happens is to have a backup that you can recover it from.

Start with this today: Back up your Mac with Time Machine - Apple Support

curiousmango wrote:

Also - side note how long is the lifespan for my 2020 iMac? I read somewhere that Apple might eventually completely drop support for Intel Macs. I'm considering just selling my iMac and get a monitor display or apple studio display.

Your 2020 iMac originally shipped with macOS 10.15.6 (Catalina). Thus the fact that it can run macOS 15 (Sequoia) means that your model has received five major version updates. As data points:

• 27" Late 2009 iMacs got seven major version updates (10.6 Snow Leopard to 10.13 High Sierra).
• 27" Late 2012 iMacs got seven major version updates (10.8 Mountain Lion to 10.15 Catalina).
• 27" Late 2015 iMacs got six major version updates (10.11 El Capitan to 12 Monterey).
• 27" 2017 iMacs got six major version updates (10.12 Sierra to 13 Ventura)

Even after your iMac stops getting macOS updates, it might be a couple of years before the last version that it can run is no longer one of "the most recent three."