Got these 3 what I think are Phoenix canariensis since around 2012, from hardware store if I recall correctly

At the beginning, they looked very leggy like this (sorry for the low quality, it's just a video snapshot, only source of photo I have of them from that time):

A year later, they looked like this (the one to the left had some damage of some sort and is to this date the smallest one, the one in the foreground as well as the one in the right background are the two big ones):

Now the small one looks like this and can still remain in its current pot for quite a while I guess:

I repotted one of the two bigger ones into the largest generally available hardware store pot (60cm, €18) recently. (I've also seen 70cm ones in another store but they charge around €75 for each, and there's no way I'm paying that much) Before repotting, roots were massively growing out the bottom:

Here it is in its new pot:

Now this is the second of the bigger ones, still sitting in a probably too small pot too (ca. 40x40x25cm):

I think overall they've been developing quite well so far considering our cool climate, I also had them standing outside all winter long except for a one week cold wave (got down to -6.7°C), otherwise they were exposed down to about to -2°C several times in some nights outside that period.

The problem is, I'm running quite low on space to put them over winter especially if they continue growing like that (got lots of other stuff like yuccas that need to be put inside too).

So I wonder if I should dare planting the second of the bigger Phoenix out in the garden experimentally, knowing it will most probably need elaborate protection in colder winters.

I'm in a rather cool/moist mostly oceanic climate on the Upper Rhine, Germany (it's overall comparable to northern coastal PNW).

Technically it's zone 8a, but with strong year-to-year variations. 2013/14 winter had a minimum of just -3.8°C (so fitting all the way into 9b), but then again other winters can have severe cold waves like Feb 2012 (probably the worst in the last 25 years):

Such cold waves are often when wind comes from the East (Siberia) instead the usual West, meaning it's very dry and hoarfrost, usually with no protecting snow on the ground at all

So, do you think it'd be worth a try planting one of them in a protected corner of the garden in proximity of the house (and if necessary build a - optionally actively heated - contraption around it in cold winters) or should I scratch the idea? (I know I won't be able to protect them forever once they get huge, but then again potted cultivation is even more limited)

I know that canariensis are unfortunately supposed to be one of the rather less frost-tolerant Phoenix (compared to dactylifera or theophrasti), often said to tolerate only -6°C (compared to values of -9 resp. -12 given for the other two), so anyone has experience with them in similar climate?