I have a fondness for the Aldus/Macromedia Freehand app, and I can still run it on my G5 iMac. But, times change. Looks like my closest alternative to not using Illustrator is some variant of InkScape. Subscriptions vs GPL. Meh vs meh.
Looong ago I used Deneba Systems's Canvas graphics drawing app. For sketching project/product ideas, block diagrams, home remodel projects, landscaping, schematics, to do lists, creating collateral graphics for products, map making, and on and on. It was perfect for my needs. Sadly, Deneba was acquired by ACD and no longer offered a Mac version. And after Rosetta support stopped, that was it. A really sad day.
After awhile, and with a lot of skepticism of it being a decent graphics software replacement, I finally went with OmniGraffle Pro by the Omni Group. It's been great. Frequent updates, good service, and available as a subscription or perpetual license.
Might be worth a look.