Hello from Finland, neighbour!
I have the exactly same combination (Explorer UL Zip + Selva 450L) and I've been a happy camper for 2 years with them. Since I mainly use my Selva 450L during spring/autumn (using Selva 120+overfill for summer), I personally use also secondary suspension with my 450L by running a long loop of shock cord through the side loops and the carabiners (it's nice that Cumulus underquilts come with really long shock cord). I also added short shock cord loops from the secondary suspension to Hennessy side tie-outs, helps keeping the quilt up in correct places (I'm a side sleeper, so that's important in cold weather).
(The only other small difference to your setup is that I intentionally decided to use cheap plastic carabiners for attaching the UQ. If I accidentally sit on just the UQ or something else happens, I want to carabiner to be the first to break.
)
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