Hello!
I can partly answer your question. I have not played much on the Lumi Keys but I have bought two of them as a gift to someone. Before giving them away I tried them to know that they worked, and I can tell you that I sometimes think that it would be nice to have a pair myself.
This is the first thing I would like to emphasise: the modularity works really well if you do not have a very hard standard on sensitivity or latency. To be able to have a two or four octave keyboard with polyphonic after touch and x-axis bend, and that can snap together and work with Bluetooth, to e.g. and iPad, is not bad at all. Especially for sketching on a familiar layout, and the keys are large enough to feel playable for me.
Good for portability with 2oct and a case and for small spaces with 4oct range. But I would get two, not one, and then the price goes up.
The lights are not important - to me - other than perhaps being good in dark environments or if you want to visualise scales et cetera.
I would not use it with Roli software synthesisers, but primarily with other sound sources, and the reliability that the Lumi has on the company software bothers me. It works with any synthesisers but is pretty closely tied to the Dashboard which to me makes it less flexible. Is it still like this?
This point may be different for you, as you mention the synthesisers specifically.
The upper/highest C key felt loose compared to the rest of the keys on both the keyboards. I figured it may be like that on all of them. Do you recognise this? To me, it felt noticeable and I hope it is not like that for you. Other than that, I pretty much liked the feel of it for being a - comparatively - affordable keyboard. I would have liked a 3.5 mm MIDI out or input for expression. But what can you do sometimes? 
I personally find the Linnstrument a more interesting and versatile instrument, beautifully built and very open. It has no BLE or battery but it is very immediate. And with a direct line of communication to the designer. It is wonderful, compact in the 128 key version and to me worth the extra ca 400 EUR compare to (two) Lumi. But it is different. And it is more expensive.the Lumi is 300 EUR, right?
So, if you would consider two Lumi and a compact and portable solution is important - for both travelling and with a small static work space - I would recommend the Lumi. Especially with an iPad or something like it and you also like playing standard keyboard.
And if you like Roli synthesisers.
If you would consider another key layout - you have a Striso? - and can increase the budget, the Linnstrument is stable and very well designed and still being developed with passion. I think instruments like the Linnstrument and the Continuum are really timeless, simple but extremely immediate and able to be complex.
There is also a hexagonal MPE instrument/controller from a French company at around the same price as a Lumi. I will look it up for you!
If you want to use one Lumi as the primary way of playing or interfacing with an iPad, I find it harder to recommend. This is up to each and everyone’s taste and need, and here mine is a broader range with more sensitivity and larger surface. And less reliability on software.
I can’t find a good thing that would be similar, though, except for the above. The Sensel Morph is not sold any more and the Arturia synthesiser with capacitive keyboard is quite another thing.
If it is for chords and polyphonic aftertouch is important, I really understand if you want to keep it.
And if you get another one, for sure get it with a case if you do not have one already! 