I couldn't really justify it an an upgrade as I've used both. In technology wise yes. Sound? Maybe? I like both.

It does pack new tech in it so there are practical upgrades like a more power efficient amp design and various connectivity functions. Great for people like me who leave the speakers on while away.
The amp stayed cool when played long periods while S530D large heatsink can get pretty warm on small rooms. But I guess you are curious bout the sound.

S530D is more forgiving and relaxed with the source while the S350DB clarity is so clean it could get youtube or low quality files sounds harsh too.

I agree that S530D trebles are very harsh no matter what volume I put. The new S350DB is quite airy and bright if compared side by side. It makes the oldie S530D trebles sound like cardboard speakers which also translates S350DB colder sound signature vs S530D warmer tones.

Given that you use it mainly on low volumes, the S350DB is definitely working good for you if you go for it. In terms of loudness I'd say S530D is more pleasant in large volumes.

Those are my personal opinions when I get to test both systems side by side at home

the S350DB is definitely cold no matter what setting you put when compared to S530D haha.
Extra obvious when you get to put them side by side
Because I felt that S350DB has better extension on the highs and the subs hits lower notes, it makes the whole middle range sunken.
Use both on nominal tone (0 treble 0 bass) and you can still feel the S350DB airyness of the highs with very subby lows with the subwoofer giving zero interference on vocals while the S530D feels very midrangey especially male vocals will interrupt the subwofer for extra warmth. But thanks to S350DB good crossover things were very clear in vocals.
Not sure what's your definition of harsh. In my case if the tweeter doesn't extend high enough to the teeny ringy ultrasonic range then it'll be harsh. Call it walkie talkie style trebles.
Quite common problem especially on Pro audio PA speakers for large halls. They have like 3" paper cone tweeters on piezo drivers and they cannot reach the bright airy sound.
Usually in PA Speakers I had to EQ the 18kHz and 20kHz to regain the airy brightness. (Now coins and keys will sound real thanks to the tweeters)

S530D sounds harsh because the tweeters are quite into the midrange so it kinda hit the 4-6kHz range which reminds me of the famous Yamaha NS10 harsh cone tweeters.
S350DB however reaches the very high highs (and even more if you turn the treble up) which can get real sharp. Tweak to your taste and you can avoid loud trebles but like I said, they are quite accurate to source so if you have poor source it'll translate very direct.
Nice to know you're getting a soundblaster ae-5. I'm pretty sure any pair of speakers would do good to you

Probably yes if you crank the trebles high enough. So far the new S series by Edifier all are able to sound very crisp and high.
S1000DB, S2000PRO, S350DB and the S360DB all able to extend really high.