I’ve always mainly ridden hardtails. No experience of the Whyte 909, but I wouldn’t fret too much about whether a particular frame will be hard on your back – the main thing that makes a hardtail not hard on you is you having decent core strength. None of the bikes you’ve suggested are going to be conspicuously brutal, but (if you don’t already) it’s well worth investing some time in core strength to keep you comfortable.
The other thing to maybe ask yourself is whether your liking climbing manifests as you surging out of the saddle and devastating your mates with awesome displays of acceleration and sustained power, or whether you’re just perfectly happy doing it. My current bike (Chromag Surface) is somewhat biased towards going down. It is perfectly happy to climb and is a pleasure, but it is not the right bike for climbing heroics, decisive attacks on the hills etc.
I lie your reasoning generally. My limit on descents is bottle, so I’m not going mad fast, which means that the hardtail makes sense. I’m disinclined to ride at the sorts of speeds or to send the sorts of lines that really need big travel, and a decent hardtail allows me to ride fairly slowly without feeling like I’m wasting the bike.
🙂