Sonar's a bit of an "orphan" these days, really...after Gibson (aka "The place on Elm Hill Pike where good brands go to die") snarfed Cakewalk, the support for it went out the window (like I said...Gibson...) and it fell out of favor. Now that it's escaped the clutches of the Guitar Deathstar, it might see some comeback action; have a look at https://www.bandlab.com/products/cakewalk
Frankly, I recommend NO plugin here. At least, not for mixing. Rather, you might find it way more helpful to get an actual hardware mixer and use that if you've not used one before for multitrack production. Fact is, until you've done this and gotten used to what TINY changes in levels, EQ, etc can do, particularly with electronic sources, mixing "in the box" isn't going to work as well as you'd think.
Second, examine your monitoring setup. If you're tracking projects with a lot of heavy bass, you won't get too much that's useful there out of a pair of little desktop nearfields. Also, if your monitors aren't properly physically aligned so that you're in their "sweet spot", things will tend to be rather deceptive, soundwise. Plus, what does your workspace look like? Is it a "boxy" room, four corners, hard walls and floors, etc? If so, acoustic treatment might be very useful in the long run. And lastly, if you're using headphones to mix...don't use headphones to mix. Unless you have some expensive AF headphone amp such as an SPL Phonitor (that has spatial correction to fix having the drivers right in your ear), you can't get an accurate idea of how your mix works due to a whole pile of physics and such that I'm not about to uncork.
Now, as for plugins that'll help...these aren't the mixing suite variety. Instead, I would strongly suggest examining your mixbus instead. With some proper EQ, program compression, and maybe a touch of spatial trickery, you can use this to "glue" your mixes much more effectively than any mixing suite might do. First up, this: https://www.kvraudio.com/product/pteq-x-by-ignite-amps which is a really spot-on emulation of a stereo Pultec set. Another useful EQ for mixbus use: https://www.kvraudio.com/product/rs-w2395c-free-neo-classic-baxandall-eq-by-fuse-audio-labs which is more of a "tone control" sort of EQ for touch-up work.
Compression: https://www.kvraudio.com/product/mjuc-jr-by-klanghelm Yep, this is a "lite" version, but tbh, it's not very "lite" as far as performance goes. Set, forget, that's it. Also, this might be useful: https://www.kvraudio.com/product/signal-noise-sn05-g-brickwall-limiter-by-sender-spike which is a brickwall limiter, goes on the very end of the mixbus to deal with stray peaks.
Enhancement: try these... https://www.kvraudio.com/product/signal-noise-sn03-g-tape-recorder-by-sender-spike and https://www.kvraudio.com/product/rescue-mk2-by-variety-of-sound And then, for a bit of stereo enhancement, try https://www.kvraudio.com/product/a1stereocontrol-by-alex-hilton-a1audio
In all cases, these go on the stereo mixbus to do final tweaks to the mix itself, rather than tampering with the mixer itself. The latter idea is sort of a "can of worms" and can result in loads more confusion than simply coming up with a good stereo mixbus for the existing mixer. There's others out there along these lines that're worth exploring, and I'd suggest that before having a butcher's at the core of the DAW itself.