I have a Klymit Insulated Static V. Was on sale for $45 on an Amazon deal of the day once so I picked it up. Pretty happy with it. Not sure what an additional 150 would bring to the table other than perhaps less weight or more insulation.
Since weight isn't your biggest concern, I have a Klymit insulated static V and while not the lightest it's very reasonably priced compared to an Xtherm and comfortable at 2.5" thick. Currently going for $52 on Amazon, R value 4.4 and 25 oz, or the 'lite' version which is 19 oz and going for $75
https://www.amazon.com/dp/B00ANRW7DI/ref=cm_sw_r_cp_api_kcBwyb9FMZQG5
I use an older Klymit Insulated Static V with my 20° backcountry bed. Fits nicely. Amazon says I bought it in 2016 so it has held together for some time with frequent use.
Mirage is a freestanding, isn't it? That's one thing I missed about the Banshee (and Naturehike I linked isn't freestanding either).
I would say tent should be the last thing you upgrade (unless you intend on backpacking), and you're right - start with the pad. Some of the modern ones are insanely comfortable, I'm a picky sleeper and I love S2S one. I also toured for couple weeks with the Thermarest XTherm and it was not bad, but also not nearly as comfortable as S2S.
If you're on a budget consider Klymit pads - here's one that weight 700g, packs very small, and has R value of 4.4 (meaning you can sleep on the snow and it will still insulate you enough). It costs around 50USD
The Klymit Insulated Static V might be a good option, especially for colder temperatures. I just bought one for an upcoming trip and have tried it out but haven't slept on it yet, but it seems to get a lot of good reviews.
The klymit insulated static v is not much more expensive (52.40 with prime) and I love mine.
I'm not sure where you live, but buying gear in Scandinavia will cost a pretty penny. You probably need an ~15F/-10C bag to be comfy (as the weather looks to low around freezing). Else look for a COMFORT RATING around freezing (32F/0C). While a $300+ bag will probably be "better" (by better I mostly mean lighter) you should be able to find something reasonable for quite a bit less assuming you don't live in Scandinavia. Both of the bags you were looking at are from reputable companies, I can't read them (google translate isn't working for the site) but if they are within the ratings above and are acceptable in size/weight they will be fine. Make sure to get a good mat if you don't have one. I recommend klymit. Hope this helps!
It seems to be $53 on Amazon and $60 for your link? Am I looking at a different one?
The Klymit V sleeping pad is on amazon prime day sale for $66 (instead of $89) https://www.amazon.com/dp/B00ANRW7DI I have it and really like it. It inflates and deflates fast.
All good recommendations.
Amazon links for recommended items:
Mine was the insulated Klymit Static V and my girlfriend's was the TNH Outdoors pad that was on sale on Amazon. Here are some links https://www.amazon.com/Klymit-Insulated-Static-Sleeping-Pad/dp/B00ANRW7DI?th=1
https://www.amazon.com/gp/product/B01LEUYJN8/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1
I have the Klymit Insulated Static V Recon and have been happy with it. I've used it down to about 0c (32f ?).
It's a little less expensive if you don't mind the blaze orange. But overall fairly competitive compared to brands like ThermaRest. https://www.amazon.com/Klymit-Insulated-Static-Sleeping-Orange/dp/B00ANRW7DI
Check out this comparison of sleeping pads with R values and other stats, it might help you find the right combo for your needs if the Klymit doesn't interest you. http://sectionhiker.com/sleeping-pad-r-values/
From another thread in this sub, snag this NAO.
These are awesome and great price. Folds up to the size of a Nalgene bottle: https://www.amazon.com/dp/B00ANRW7DI/
I've used a Klymit static V insulated for a couple of years with no issues. Comfy and packs small.