I just completed my 2020 sport amp and stereo upgrade. A couple items to watch out for.
1 speaker box max outer dominion is 13x8.5x6. That limited you to a 8” sub.
2 there is a post under the passenger seat. You either need to cut it off or prop the sub box up to clear the post. I used 5/8 board and was able to drill a small hole (not through) because of the post.
3 think of how you want to upgrade your system. Are you going to install a 4 or 5 or 6 channels amp. Because you may be able to install a 4 channels amp by tapping to front speaker, swap out the door speaker that can do 30hz and use the amp to split out a separate signal for the 3” mid/tweeter in the dash. I did that for my wife’s 2018 crosstrek and sound much better than just strait speakers upgrade.
4 search Amazon and see if the powered sub works for you. That way, you only need to tap the front speaker level for the powered sub. I did not go w that route.
5 think how you are going to wire all the signal/speaker and power/ground wires. Plan it out first.
6 try upgrade all factory speakers w higher sensitivity and lower frequency responses first, you may be happy with it and no need for the sub upgrade. you are going to replace the factory crappy speaker any way. Unless you have HK upgrade package, the front door speaker is 6.5” not 6x9.
7 Also, the top cover for radio is pain in the @$$ to remove. IMHO, the oem head unit is good enough and put out good enough level sound with better speakers.
Good luck!