Can’t figure out my moody Dracaena

I've had this character for 10 years in my life, we've been through a lot together and for the last year or three it's had to contend with my renovating the apartment. I have tried, however, to accommodate — the entire living room is kept warm for the plant family (pictured) of various Dracaena plants, and they do okay but the big guy I haven't figured out. The latest issue I am observing — everything else having improved as I've learned its habits — is droopy leaves, as you can see on the photo. The top shoots go straight up then everything below those just drops straight down. I think in plant science this is called fluid pressure? Just a hypothesis…….

Some stats:

• Temperature is 64-68F (18-20C) at all times
• I try to maximise humidity but the building is porous and the winter here is so dry all the moisture is sucked out of the room in mere hours — I am struggling to keep it above 30% RH in the room, have to wet the same towel to hang and dry at least once a day, and RH then climbs to above 40%, that's most I can do, really, before I acquire a humidifier
• The plant sits in a true terracotta pot with a hole at the bottom, and I make sure there's no water pooling in the tray after watering the plant
• It's watered every three weeks, roughly
• Misted every week or so; have to say misting has proven to be a "placebo" if I have learned anything
• Soil mix is one I'd consider airy and well-draining — close to half is perlite, the other half is LECA mixed with regular indoor potting mix
• There have been no brown leaves lately — what brown leaves there are, it's from way back when I had it in a different pot that had LECA at the bottom, something I figured wasn't a great idea so I had the plant re-potted last spring
• Installed LED (pictured) because I am 59* northern latitude in Northern Europe (low sun and sunsets around no later than 5pm since November)
• Plant is by a south-western wall, there's balcony and a 2m² window (pictured), but the latter opens to NW and the balcony has tall railing, so there's just no way to get enough light until summer, no matter where you place it and how you spin it, literally

Any great ideas, anyone? I am like most self-taught — maybe I missing some obvious signs? I think it's been thirsty, as I have other images after watering and it's looking much better on those, but then there's the LED thing which is relatively new — have I made a mistake trying to give it more light? It's not a lot of light food, if I have learned anything about PPF, 37μmol/m²/s @ 20cm, 3 meter 4000K 36W strip of garden variety (pardon the pun)…

I have his/her tiny little sibling in a starting pot mix you can spot there, and there's also the well-camouflaged middle sibling behind (almost can't see it behind all the foliage), these thrive just a tad better as good as the big guy but you can see they're a bit droopy too.