I, too, find it very daunting when I'm doing a new garden, being a rank amateur at this! My current garden is totally different from my last one, and it took me a long, long time to figure out how I wanted to landscape it. I'd always done borders around edges previously, and I knew that would look really terrible with the kind of slope I have, so I was basically forced to try curves and a layout that placed taller things more centrally - which felt like a great departure from anything I'd tried before. I'm really pleased with how it's coming together now, but I have moved some plants 3 TIMES in as many years!! I suppose that's really what I want to say: that ideally it'd be perfect first time, but that (thankfully, in my case) most plants are forgiving of mistakes and will tolerate being moved!!
If I were in your shoes, I think I would spend this year mostly reading and visiting other gardens and actively planning, planning, planning. You have enviable, enviable amounts of space to play with, and will do the most amazing things with it. But it is bound to take a little time to really think through what you want, particularly if you are even considering a design that is based on subdivisions into some kind of rooms, or a design that isn't entirely centrifugal (and with the amount of space you have, you're pretty much forced into thinking differently about that, I would have thought!!).
Visiting places that have similar amounts of space and seeing what you do or don't like is something I really enjoy. But sometimes I think that we often see larger gardens with 10+ acres and there is a job then to work out how to apply the inspiration at a smaller scale. I went to York Gate Gardens a short while ago, which I think is about an acre in size, and was so inspired by what they had achieved, and by how instantly translatable it was to my (smaller) garden. So getting out to some NGS places, and some of the many beautiful gardens near you (there are lots to the West I think?) might be useful.
I suppose there are some questions here about time too: what you need now may be slightly different to the garden you want in 15 years' time, so there may be some parts that you need to keep to suit current needs and some parts that you develop with an eye to them looking good in 5+ years. I'm thinking of things like hedging or topiary, which can take a while to look good.
To get a gardening fix, what about sowing some biennials for next year? It's nearly time to do things like foxgloves, lunaria, sweet rocket etc! Also, planting up pots that you will be using long term might be nice. ALso, you can't go wrong with loads of repeated box balls or yew topiary, and they are so much cheaper to grow up and on than to buy.