As others have said TDS on it's own is pretty meaningless. It really depends on what makes up the TDS which a TDS meter won't tell you.
That said, I have hard water (GH12/KH5 and TDS out the tap is ~300). The tank runs ~340 TDS and I add a reasonable amount of ferts. So whatever makes up the TDS is there in a reasonable amount (more than you would expect)
Edit: Re-read and it was confirming the reading. I see it did drop to 288 after a water change. Which means you (or something) is added quite a lot of something to the water post water change
The confusion really is that if you have TDS of 542 and replace 70% with tap water that has TDS of 54 your tank should drop to somewhere near 200 and you are saying it is still that high after a 70% water change?
Have (can) you calibrate your TDS meter? I have had some from Amazon here which are basically random number genertators....
There is no calibration option on my TDS meter, but it's consistent. Tap water always 54 (+/- 3), Bottled Mineral Water always 108 (+/- 2), tank water within 10 TDS of where I left it the day before
When my tank's TDS is 228 after a 70% water change with 54 TDS water (tap), and I then add 200ppm in ferts + Prime + Excel + GH buffer. I then add crushed coral, and my tank settles at 515 TDS, I'm not surprised or concerned.
I'm adding Excel every day, along with alternating micro and macro ferts, and every day my TDS is going up around 10-15ppm, I'm not surprised.
I'm sure I could bring my TDS down to around 300 very easily with just one more water change. I suspect after several weeks it would have creeped back to the 500+ range, unless I did two water changes per week, or reduced the salts I'm adding.
Right now I can't see any reason to do either. The salts are in their to enhance plant growth, I see no reason to dilute them with water changes or stop adding them.
Hey, I've only had the TDS meter a few days, perhaps I will notice something more sinister happening over time, but right now I'm not concerned.