A few comments...
A basic premise we use is that any nutrients we take off our ground need to go back on the ground. So if we feed hay and or silage made from our own grass, the manure from that needs to go back on teh fields.
We can't produce straw here, so we basically decide on a proportion of the muck that derives from the homegrown hay / haylage input and a proportion that derives from the brought-in inputs, and the latter is allowed to make its way to the veg growing areas but the former must go back on the fields.
Another comment is to ask did you make sure you used the guidelines for horned beasts; they are necessarily very much more generous than the general beef cattle ones. And there are specific guidelines about feeding areas for housed horned beasts too.
Another is to say that my experience is that beasts such as Galloways, Highlands, Blue Greys etc do not take well to housing. If you are going to house them, you may find you need to shave along the spine to allow them to lose heat. And you may need to ration their inputs strictly to prevent them becoming overfat, and potentially having calving issues.
In my experience, cattle will eat fresh clean straw from the top of their bed but will not rootle in soiled forage.
Our approach to deep littering cattle is to remove or cover all soiled straw twice a day, so that udders are always on clean, dry straw. One year when this was not accomplished, we had an issue with mastitis the following summer, from bugs picked up during the winter from the strawed field shelter and plodging through the mud around it.
We kept two cows - similar sized animals to yours, maybe slightly larger - in a pen 20' x 10'. They had a covered feed area and an uncovered loafing area outside this pen; they used this pen only for sleeping and would shelter in it when it was raining and they were not in the feeding area. Surface soiled straw was removed twice a day, fresh straw added to generously cover all wet and soiled straw twice a day (sufficiently generous that they would eat some of the straw before it became compacted and or soiled). We used less than one bale of straw a day but probably a bit more than 1/2 bale a day on average. Litter depth grew by approx 1' per month - but that won't be linear, as the older stuff will compress more over time.
In short, we would expect that pen to need mucking out (whether fully or partially) once during a four-month winter, or they would be stepping out over the side walls by the end! lol