My progression of Yamaha arranger-type keyboards would be the PSR-500, PSR-510, PSR-520, and now the PSR-E433. I am not counting the Yamaha D80 home organ and DX-7 synth I owned decades ago, as those are different categories of keyboards.
I actually sold the DX-7 synth to buy the PSR-500, which was the first home keyboard that I saw that I felt was superior to the DX-7 in many ways. And it was the first moderately priced keyboard that offered the ability to combine sounds, set the octave of each combined sound independently, add effects like reverb, and split the keyboard -- and do all this with a useable amount of polyphony. It also offered a multi-track sequencer similar to that which is on the current PSR-E4-series, though with only about 3500 notes instead of about 19000. It was a very good keyboard for its time, and if you find one in good shape for a good price today, it would still be a nice starter keyboard that would grow with you.
The PSR-510 was a kind of unique keyboard that added some features and sounds to the 500, including a rotary-encoder to set the parameters. Strangely, however, Yamaha reduced the available memory for the onboard sequencer to about 1600 notes or so.
The PSR-520 brought the line to a new level, with a multi-feature LCD display screen, higher quality sounds, and DSP effects. But, also strangely, Yamaha reduced the number of melody tracks for the onboard sequencer from 5 to 2, but I wasn't doing much recording at the time, so that didn't bother me much, then. Also, while the 500 and 510 could combine two voices for the split/left side of the keyboard, the 520 (and, as far as I know, most successor keyboards) only provided one voice for the split/left side.
I got the 520 back in 1996, and I didn't get a new main keyboard again until 2013. One reason was that most of the Yamaha keyboards in this price range after the PSR-5-series keyboards were discontinued only provided two registration buttons per bank, which to me, is very limited. Only when I saw the PSR-E433, did I find that they finally included four registration buttons per bank again.
While the PSR-E433 does not have some of the features of the PSR-520, such as multi-pads and a variety of DSP effects, it does add features such as filter and envelope generator control, live control knobs, DJ patterns, arpeggios, and USB outputs. I recommend it and any of its successors to anyone who wants a great sounding keyboard at a great price.