Hi,
I just have one folding electric bike. There are two places I keep mine, both convenient as I prefer to sleep down below. (1) Chained and padlocked through offside rear wheel (with slack to allow door opening) with two further locking wire straps through the bike wheels and frame. (2) As (1) but attached to the rear towing hook. I also have a Halfords cover. My reckoning is that if anyone wants to nick it - they will have to be the quietest most skillful thieves to be able to nick it from a few feet from where my head is.
Like yourself, I have quite a few hundred quid invested and would hate to see my bike stolen. I think it is well worth spending money on good quality locks and the time spent securing the bike(s).
Mine travels folded inside the van with the rear seat pulled forward to stop it moving about - no bike rack - bike too heavy.
Happy vanning and happy secoure biking. Bob