Sadly, the days of optimization are gone. And if you think your 2018 i5 has aged poorly, wait until you experience the torture of the 2020 i3 MacBook Air. A stone tablet and a chisel are faster. Completely unusable on Sequoia.
So, options. Aside from removing all extra fonts and running one app at a time, you have very few options. The system is what it is. 8 GB of RAM with a CPU released in 2017 and customized to fit Apple's thermal limitations. Check the /Library/LaunchDaemons and /Library/LaunchAgents folder for startup items you don't need. You might be able to suppress a few background processes this way. Check System Settings > General > Sharing and make sure you don't have any extra services running.
You can consider rolling back to Sonoma as you should have slightly better performance. And since the 2018 MBP cannot participate in any of the Apple Intelligence features, going to Sequoia really doesn't offer many compelling features for your hardware class. After all, if you roll back, you are still getting the latest Safari (if that is your browser of choice), and Apple will continue to support Sonoma through the Fall of 2026 (assuming to continued pace of annual OS releases and the current minus two support model).
Note, today's MacBook Air will run circles around your 2018 MBP. If you eventually get to the point of replacement, make sure you are considering the Air as this will provide a cost savings over the Pro.
Hope this helps.
