I’d say a 4 / 3 nights split between North and South is a perfect balance. Presuming you will have a car you could even do 3 and 3 and have one night in the "middle" to break the journey south - most visitors just drive past Mid-Wales but it is quiet and scenic!!! You could have a night in Aberystwyth on the coast or Machynlleth inland, although the drive between North Wales and Pembrokeshire is only about 4 hours though and lovely.
In North Wales (which is what I know mainly know about!) everywhere is a possible day trip so definitely stay in a central location - maybe Betws-y-Coed or Trefriw which are cute villages and are both literally in the middle of North Wales, have good accommodation choices, and have easy access to the rest of Eryri (Snowdonia) National Park and Pen-y-Pass car park for Yr Wyddfa (Snowdon). They are also both near the A470 road which is your route south to Pembrokeshire via the A487.
Yr Wyddfa (Snowdon) itself will probably take the best part of a day up and down (5-6 hours at a good pace) regardless of which path you do so allocate one of your days for that. The trails called the 'PYG track' up and 'Miners Track' down are a nice combination of scenery (both start from Pen-y-Pass).
The other day or two you could do a nice drive through the mountains and back along the coast to see places such as Beddegelert, Caernarfon, Conwy and Llandudno - a sort of 'greatest hits'! Or do another hike.
Towns like Conwy are really nice to visit but compact and you can always visit in an evening combined with dinner, and they are quieter then anyway. The castle would be closed but you can get up close and still walk on the town walls so not missing out on anything!
If you did want another hike apart from Yr Wyddfa (Snowdon) then you are spoilt for choice but personal favourites with amazing scenery are:
- Cwm Idwal (1-2 hours),
- Llyn Crafnant and Llyn Geirionydd near Trefriw (4-5 hours),
- Aberglaslyn and Cwm Bychan near Beddegelert (4-5 hours)
- Cwmorthin / Tanygrisiau near Blaenau Ffestiniog (1-4 hours - you can
see an old abandoned mining village)
You could probably squeeze two of those hikes into a day.
Oh and for mines in North Wales consider: Blaenau Ffestiniog (the town itself is nothing special but you can go underground at Llechwedd on a tour), Dinorwic Quarry in Llanberis (good place for walks), Sygun Copper mine near Beddegelert (underground tours), Great Orme Mines in Llandudno or for the ultimate experience a 'Go Below' adventure activity underground near Betws-y-Coed