Alex, these are AWESOME photos... for your 'first attempts' - absolutely stunning work.
The ultra wide perspective is very cool.
Very quickly, I did some basic calculations, and realise your 'black glass' is WAY WAY WAYYYYY darker than even 3 x ND8 filters stacked.
I have 1 x ND8 (72mm) filter and a 1 x CPL (72mm) filter - which I sometimes stack. The ND8 (and if necessary stacked with CPL) helps in some situations eg with waterfalls in broad daylight to get a long exposure... but can't give the effect like you get.
Inspiring work... I really like that the colours and tone of your photos are so 'strong' and yet natural. (I do use HDR on occasion, but very carefully - not to overcook it).
Cheers... thanks for sharing.