If you want to get an equivalent action to your Motif ES6 or 7 (FS action), you’ll want to get the MONTAGE 6 or 7 (FSX action).
The Motif ES (2003) represented the top-of-the-line synth, the MO series was developed as the cost down version of the Motif-series. Half the price, half the weight, all of the ‘sound’.

The MONTAGE represents the top-of-the-line synth, today, the MO(DX) series was developed as the cost down version of the MONTAGE-series. There are many parallels.

The FS and FSX actions feature Channel Aftertouch, and are virtually identical synth actions (found in both the 61 and 76 key models).
It is the same lineage as the action first developed for the legendary DX7 (and when you consider it was also used on the Korg M1 bank-in-the-day, it becomes one of the most popular synth action keybeds in history).

The only difference in the FS and FSX action was the FSX action is more environmentally friendly (when 30 years from now your synth returns to landfill ...it is more a “green” thing than anything else).

If you know the Motif ES action - and after 15-17 years playing it, you most certainly do - do yourself a favor and get the MONTAGE.
I cannot describe actions to anyone... Because you have a solid reference point (the ES), I can tell you, you already know what the MONTAGE 6 and 7 feel like, exactly. If you are going to get a MODX6 or 7 please make sure you visit your local Yamaha dealer and play it. It’s the only way to know... action is like trying on shoes, you have to test how it feels (that’s part of the process).

The MODX and MONTAGE are related, like the Motif and MO series were related, sound-wise the upper and lower cost models are very similar. There is a similar “half the price, half the weight” ratio thing going on. The main differences are in the touch (keyboard action/Aftertouch), in the encasement, (aluminum versus lighter weight material), the number of analog and USB audio Outputs. More front panel Knobs, Sliders and lighted buttons, plus a Ribbon — so the user interface differs some what.

Asking what the action is like... is best answered by actually playing it. Hope that is helpful.

Thank you very much. Your answer to my question is for sure the proper answer.

Unfortunately I have a personal issue that may prevent me from doing the most reasonable thing to be done as you say (try it in a shop).
That's why I was trying to see if I could have somehow any kind of "descriptive" comparison of the key action.
Let's skip aftertouch. I may survive without aftertouch.

Basically my motifES had an FS keyboard, where keys are not fully weighted, but anyway they are not completely weight-free. So I would define may motifES keys as semiwaighted, while instead Yamaha does not: it defines motifES keys as FS and modx keys as semiweighted.
What does it mean?: that the modx keys are less seimiweighted than the motifES keys?

Unfortunately, the modx was perfectly fitting my needs for Weight (6kG) and Size (about 10cm less), while the montage still has weight and size of my current motifES.

Massimo

Keybed feel is subjective, and really has to be evaluated in person. A given keybed might feel great to one person but not to another.

I've had both the FSX keybed (in a Motif XS6) and the cheaper version keybed in a MODX7. The FSX does have a nicer "feel" to my fingers but the Montage was too expensive for me so I went with the MODX instead. I'm happy enough with its keybed for half the cost.

If you've had the FS or FSX keybed in a prior synth (Motif ES in your case), you'll get the closest feel to that in a Montage. While both are considered "semi weighted" the FSX keybed has more weighty "feeling" keys (to my limited memory... bear in mind I sold my Motif 6 years ago) compared to the MODX.

I know it's hard these days with the pandemic, but trying keyboards in a store is the best way to determine which works best for you.