I think it's a reasonable strategy to contribute the max possible to your pension, and then overpay the mortgage with additional. Time in market brings returns, and once the year has passed you can't get the valuable relief.
You can max out last year before October 31. So whenever you start make sure you do the previous year. Or maybe do the previous year this year anyway?
The regular advice here is to get your mortgage under control before paying pension. You would be at 2 x LTI, and approaching 50% LTV for the new house (assuming you use all your savings)
If I was in your shoes I might wait til the house costs are clearer, and mortgage rate is known, before locking away the funds in pension.
You are on a large wage. How secure are your earnings? What does future earnings look like?