My guess is that 100 lumens per watt LED bulb is still in the laboratory, and likely costs a mint. Exactly. I would also point out that a $5 LED with an efficacy of 150 lumens per watt isn't very practical for home lighting when its output is only 50 or 100 lumens. A 75 watt incandescent lamp produces about 1200 lumens. Think about how many 75 watt incandescent lamps the usual kitchen fitted with can lights requires, and add up the total cost for LEDs. Ouch. And before you dismiss this, consider that blue LEDs cost about that much per copy, 10 or so years ago. On the other hand, as I said above, LEDs really shine (groan) in low-output applications, where incandescent lamps and even fluorescents have poor efficacy. My point is that we have to consider what technology is appropriate for a given application. It's important to distinguish brightness and illumination. You need brightness when you want a light TO BE SEEN. You need illumination when you want to SEE BY a light. LEDs have high brightness but poor illumination qualities (low lumen output). Because of the brightness, it can actually be painful to look at an LED. This makes them appropriate technology for situations where the light source must be seen at a distance. They're good for indicators such as automotive taillights and traffic signals. Because they emit light from a very small area, LEDs aren't very good for lighting up rooms. However, because incandescents and even fluorescents diminish markedly in efficiency as they're asked to produce lower illumination (lumen output), LEDs can be an acceptable alternative. (But find a way to eliminate ballast losses, and fluorescents could still win at this game.) Fluorescents have high illumination (lumen output) but low brightness. They produce a smooth, even light. It is not usually painful to look at a fluorescent, because it emits light from a large area. This makes them appropriate for lighting up rooms, but not so good as indicators. Rather than follow up on this any further here, I would direct you to Don Klipstein's outstanding lighting pages. Don follows lighting developments, especially in LEDs, pretty closely. This page has more on why LEDs are so well suited to flashlights and traffic signals. Anyone with an interest in lighting, especially high efficacy lighting, will most likely find Don's pages very interesting.

If you look closely at the photos of the fixtures, you will see that the Nora Cobalt is a "retrofit" type fixture (like the Halo the OP originally asked about) where the driver, heatsink and LED board are in one compact "trim" unit that is installed in a standard metal housing used for various kinds of fixtures. If the fixture fails, the "trim" unit would need to be replaced. The Lightolier LyteCaster L3 is a modular design with the driver (step-down transformer and current stabilizer) mounted in the housing and the light unit and heat sink fits into the "trim". If the fixture fails or the color dims, the appropriate part can be replaced. The parts can also be upgraded. These fixtures need a more substantial heat sink because the COB chips are more compact than the typical SMD chips. The Nora Cobalt is intended for residential use and the Lightolier LyteCaster L3 is intended for residential and light commercial use. I recommended them because they are both more powerful than typical LED recessed fixtures, are more deeply recessed, have an adjustable beam angle and use a superior COB LED. The L3 also has a great wall-washer for a more uniform light on a wall but it costs another $20+. There are no bargains in good lighting.

Both lights and LED compatible dimmer were purchased within the last few months, so it's latest technology. There are only two on this dimmer and both flicker (by flicker I mean an intermittent dimming, not rapid blinking). One suggestion I saw was that, since they are wired in series, if the first one is having power fluctuations, it could impact the second one. My first idea is to swap positions and see if both still flicker. The problem is that this issue is really hard to reproduce. You need the lights on at full power for 2 or more hours. Because the lights are so bright, we don't usually have them at full power (and there is no flicker). The reason I suspect the Thermal Cut Off Device is that after 2+ hours on full power the lights do get pretty warm. Maybe this behavior is by design?

Or better yet, replace the existing trim and bulbs with some Juno Lighting trims (example: Juno Recessed Lighting 27HZ-WH) and compatible LED bulbs. "Which is a more modern and luxury option?" IMO, separate trims with LED bulbs are the best option. I tried a bunch of retrofit integrated lights like the one pictured above by @3onthetree and didn't like any of them. "either way, we will have to replace the LED by an electrician, correct ?" No, you can install many retrofit trims and bulbs (or integrated reftrofit lights) in existing housings yourself. If you're not sure you can do it, pick one existing light, remove the bulb and trim, pick up a cheap retrofit LED light at Home Depot, and try installing it.