Oh dear :( Think my rose is in diffs ....

Hi all
I bought what was supposed to be a barerooted standard rose from the cheap German shop we all love to buy our food at of late LOL, & sadly no doubt the price was a warning that it might not behave as a standard. However, no doubt too, my care of it hasn't helped.

When I got it, it did indeed look just like a little standard bush at the top of a bare stem, but within days of planting it up, it suddenly decided to put up 1 stem to the skies. I didn't actually notice it until it was a few inches shorter than it is now, & because the lower growth has all but died off, I was reticent to prune it back in case this might be the only growth it could ever have. Sorry I couldn't get it out from the rest to make it a bit clearer but I'm disabled, so things tend to sit where my brother puts them until he's next around to move them :(. Hopefully you can see it against the backdrop of the other rose behind too.

So, I'm looking for a bit of input as to what your views are - leave it alone & prune back later in the year, or prune it now? Given it's not in that big a pot either, I'm concerned it'll take off again when it's repotted & just needing a bit of advice on what to do & when.

Thanks
Romayne