If they were offering a decent rate (especially for niche skills) they wouldn’t need to worry about you going elsewhere.
They would kick you out on a moment’s notice if they were to run into difficulties… Just take it and make sure there is a notice period either way so that you can go to Switzerland if you get an offer. DO NOT sign a six month contract where they can give you a week’s notice but you can’t get out: I’ve seen too many of those.
It really annoys me when companies expect loyalty from their employees and contractors but aren’t prepared to offer any loyalty in return.
Take the job and then leave if you get an offer from a higher bidder: it’s what contractors do and they should realise that.
Sounds cynical, but I’ve been let down by enough companies to realise that my only work loyalty is to me. It’s the way it has to be now, sadly.