Its never to early to think/ scheme for the coming year, what else is there so do during dark wet winter months.
Thinking about the last paragraph of your post, that rotation is your key, i suggest two things.
First if i was you split your 13 acre park in 2 to allow you to either rotate through three parks with one set aside for hay, or rotate through four parks which allows the ground to freshen more between grazings. I know you said you did not want temp fencing/ cell grazing so if you can bang in some perminant fencing, its not hard to do and it will pay for itself in short order with more hay and less worms.
secondly there is an old crofting saying that sheep should never hear church bells in the same park twice. Thats a good rule of thumb to evaluate your stock density against how they are getting through the sward in your parks, if you use the three park system and the sward is done or worse being damaged by rotating on this period, then you can move to the four field. If you try the three park system think about when you will close to grazing the hay park, i close mine at lambing so the field is clear of stock from march onwards in order to clean and grow. That works well at the moment, though as my numbers are rising and we are having a series of wet winters which cause lots of damage to the parks i am thinking of moving to a sacrafice field system or even partil housing post tupping.
anyhoo thanks my tuppence, good luck