A tricycle.
Seconded, actually. I've been known to use three wheels in winter for exactly this "might be ice, but probably won't" reason.
An extra wheel with a normal tyre adds some rolling resistance and aero drag, but it's not as hard work as a bicycle with winter tyres. While prolonged patches of sheet ice can result in loss of traction and/or steering control, you do generally stay the right way up.
The disadvantages, at least with a tadpole recumbent, are colder feet compared to an upright[1], and that your face is down in the beam of everyone's dipped headlights (which is also gritter height).
[1] Demonstrated to be postural (rather than air cooling or whatever) while sitting around on a stationary trike awaiting puncture repairs on group rides: Almost instant increase in foot warmth from unclipping from the pedals.
Not really an option,
a) I don't own a trike
b) I'm coming back on trains that only allow 2 wheeled cycles...
c) I'm skint.
I've never used spiked tyres but I have ridden in bits of Polish winter on hybrid knobblies. Never a whole winter, too cold! People do, though, and without spiked tyres. And certainly not 600km in three days either. Are you expecting (the possibility of) thick sheets of ice or isolated patches and frost? If the latter, experience and observations says you probably don't need spikes. So I guess a lot depends on how well ploughed and gritted the roads.
My big concern is the patch of slippery stuff on a corner when going round a corner in the dark, and not knowing it, and finding myself laying in the gutter unable to reach the panic button on my inreach, when I have no signal on the broken phone in my pocket...
Or shall I just stick 8 bar in them and hope for the best?
8 bar? I only ever have to talk of pressure in tyre terms, so that means I had to convert this – 116psi. Surely if you're planning to use normal tyres you should be letting the pressure down for ice and snow? You don't say what width they are, but if they are sufficiently narrow to justify those pressures, they're probably not going to be much good on ice, I'd say.
On this bike it's the 42-622 Marathon Winters. They only go upto 5 bar, the 30-349 Schwalbe Winter's on my Brompton go up to 8 bar.
At max pressure they run a lot smoother, the idea is you run them at 5 bar when it's not too bad, and drop it to 2 bar when there's more issues.
So my thought of 8 bar, was a) remembering the brompton pressure not the 700, and b) run them at max operating pressure so as to minimise rolling resistance.
Spikes work, but at a price: as Kim says, the rolling resistance is extremely high. I only ever used them for short commutes, and I think the longest I've ever ridden on spikes in one go is about 25 miles. I wouldn't want to do 600km on them!
Longest I've done on spikes was 90km, through the Ardennes' last winter. A couple of days later in Germany the temp had gone up 10°C, and I had to hunt puddles on a descent to keep the studs from getting too hot...
I've long been curious about the Conti Winter Contact, which claims to be a non-spiked winter tyre (https://conti-tyres.co.uk/commuting-touring/top-contact-ii-winter-premium), but not curious enough to invest in any (these days I just don't go out when it's icy).
I have no choice, I have to get to work, hence having the spiked tyres in the first place. Having them makes me think that 600+ km across Germany in winter is a good idea...
I used to commute on my Schwalbe Marathon Winter tyres in the snowy weather. They were only really necessary for 1.5km of my 11.5km commute (the roads after that were clear). Absolute confidence where it was a struggle to even walk.
Inflated to the upper limit they aren't that bad, probably only 10% slower than non-spiked tyres (pulling a number from thin air).
Neither of the Schwalbe winter tyres (https://www.schwalbe.com/en/spikes.html) have spikes down the centre line of the tyre, there's only minor contact of the spikes when riding normally if the tyres are inflated well.
But, 600km over 3 days on spiked tyres? No thanks. I'd go for normal tyres + caution or bin the idea completely if the weather looks awful.
The thing is, if I get 300km into it, and find that in front of me is snow and ice, my only option would be to turn round and find a hotel. On Christmas day, in the middle of Germany.
My route takes me round the bottom of the Harz mountains... all the way to Leipzig.
J