Phonics needs to be taught discreetly, to the two year groups separately (and possibly to smaller groups within the year groups, depending on the scheme used by the school and the resources available).
The EEF Guidance on improving maths at EYFS & KS1 includes a recommendation that there should be a regular maths input focusing on a specific area of maths (I can't remember the exact words - it's well worth reading). They say that children won't be able to make the most of the mathematical activities you put out for them to engage with in the continuous provision unless the activities have been modelled to them. Personally, I've never managed to teach mixed age maths to more than one year group at a time and be confident that I'm properly meeting the needs of different ages pupls - I'd need to teach each year group separately.
Your greatest challenge is that you are running two different curricular alongside each other. The ELGs for the end of Reception are very different to the year 1 NC expectations. Knowledge & Understanding of the World is not the same as Science or Geography.
If it was me (I've taught mixed year 1/2, but had SLT oversight of EYFS), I would have a theme or topic for half a term, chose books to fit in with it and then plan as much as I could to fit in with it. In Reception in particular, your environment and continuous provision are crucial. Your year 1 pupils will benefit equally from the chance to engage in child initiated learning. Will you have any kind of passport system, where pupils know the activities they need to complete each day?
Your other challenge will be that you will need to run a two year cycle of themes as half of your class will have done it all last year. But surely the school has given you the overviews and planning created by your predecessor?
Good luck - it's my idea of hell!! When you've got it working well, you'll be able to give your year 1s a really strong transition from the EYFS curriculum and way of working to the KS1 curriculum and expectations, so they are ready for the demands of year 2.