I note that I have come into this thread a little late but I have also noticed you don't intend to embark on your tour until Sept. Also I don't know whether you are a person who books things well in advance and like B+B and where is the pub kind or an " I have got a general idea where I'm going, here is a good place to stop and anywhere is somewhere when I arrive, I don't carry the kit for nothing sort." As I tend to be.
Maybe these suggestions are completely irrelevant to you - but here are a couple of ideas.
It occurred to me that if you made your way to Shrewsbury, you could then use the Mid-wales railway to head towards Swansea, where it ends. This has about five trains/wk day starting at about 7am and the last at about 6.18pm. This passes through some very beautiful parts of Wales and many stations (halts) are request stops. So long as you avoid the first and last trains - particularly the first on Mon and last on Fri you are very unlikely to have any problem with your bike. It is a very personal service, to the extent for example, that as a local, I have several times paid them on my return because neither they nor I had change on the way out. I think, but am not sure that if, on the spur of the moment, you wish to get off and tarry a while somewhere they will let you board the following train and continue. If you wish to alight at a request stop you must make sure that you make your request to the ticket collector/guard on boarding - because if their is nobody on the platform it won't stop. You might find this a refreshing and interesting way of heading towards Swansea.
Also considering my previous mention of the disruption to the cycle route N of Machynlleth - but several ways round (if you happen that way).
As you move North of Llanidloes - following the cycle route round the western side of Llyn Clywedog; which is a lovely lane with (according to your camping stylee) a number of great places to discreetly stop. This is much easier to cycle than the B4518 road round the Eastern side of the lake, carries little traffic (not that their is a tremendous amount anyway) and IMO is far more scenic and interesting. Continuing along the cycle route, you then join the B road and turn left, slightly N of Staylittle.
A mile or so along the cycle route turns left and crosses the Plynlimon hill range, via Dylife to Machynlleth.
Instead continue straight on - more or less due N for Llanbrynmair. At Llanbrynmair continue N across the middle of Wales motorway (A470) over a staggered cross rd (L then R) up the valley, until you come to the A458 a mile or so W of Malwyd - turn L towards Malwyd and then right (on the middle of Wales motorway) to Dinas Mawddwy. This is the only crap bit, there is a lane which misses some of it; but whether it is worth it or not....
At Dinas Mawddwy turn R up the valley for Llanymawddwy continue N past this, up the incline, then bearing L through the Bwlch-y-Groes pass just before you crest the slope (a very satisfying mound of rock to stand upon, I find...).
Heading down the valley (NNW) to Llanuwchllyn - a little to the S of the southern end of Llyn Tegid to join the A494 (but you are returning to Manchester- so here is a particular place of choices for you.). However to continue this variation on one of my commuting routes:
Turn R onto the A494 at Llanuwchllyn (towards Bala) and turn L after 3/4 of a mile or so onto a lane. Here you ascend E into easier going very open hill country, unlike any other terrain on the described route so far. Perhaps reminiscent of the N York's moors.
Head across here towards Trawsffynydd, Turning R as you re-join the cycle route and descend to Trawsffynydd (if you find a Landranger map of the area blowing in the wind hereabouts - its mine!). Again crossing the middle of Wales motorway.
An interesting variation on the route here, is to follow the cycle track round the edge of Llyn Trawsffynydd. Which takes you across a number of loch and sluice gates right round the back of the nuclear power station (now being de-commisioned), where you can look in through the windows of the main reactor building. Thus feeling the experience of what Fukushima used to look like. Except this power station used to burn the more dangerous graphite fuel rods. It has had nuclear "incidents" which have dumped nuclear waste on the surrounding landscape. The bottom of Llyn Trawsffynydd has significant amounts of heavy nuclear sludge, which is gradually washing down the Porth Madoc estuary into the Irish sea. An eyrie, awe inspiring place I find. So incongruous amongst those hills. However I digress.
Continuing down the cycle path, onto the lane and descending to Gellilydan, at the head of the Porth Madoc estuary, again depart from the cycle route. This time to avoid the pointless and grueling switch back hills from Rhyd towards Beddgelert. Instead turn L along the Southern side of the estuary towards Talsarnau and Harlech on the A496. Before you get to either of these places, turn R over the toll bridge (free to cyclists - especially if you greet them in Welsh and stand in the middle of the road bimbling about going through your bags to find 10p while you hold everyone up!). shortly after this turn R for Penrhyndeudraedd across the A487 onto the A4085 crossing the steam rail line, towards Beddgelert.
Before Beddgelert at a hamlet called (its on the map I lost)... before Nantmoor is a lovely little lane on the right, this skips Beddgelert and goes round the back of some old copper mines up a very quite valley. It comes out just past the head of Llyn Dinas and below the foot of Llyn Gwynant crossing Nant Gwynant onto the A498 Beddgelert - Capel Curig road. Turn R here towards Capel Curig.
Across the end of Llyn Gwynant and beginning the ascent of the Llanberis pass towards Capel Curig.
At the turning for Llanberis, on the L, you again have a particular decision point. You could continue along onto the A4086 for Capel Curig and then turn R onto the A5 towards Betwys-y-coed for a short distance before turning L onto the lanes towards Llanrwst and then L onto the B5106 to Conwy where you could use the cycle path or board a train across the top of N Wales.
Alternatively, you could turn L on the A4086 and continue your ascent of the Llanberis pass, crossing the gap through Pen-y-pass and descending via Llanberis towards Bangor. Here you again have the choice of using the cycle route along the N Wales coast or boarding a train.
That is the end of my suggestion. It is a variation I sometimes use on one of my fairly regular commutes by cycle to a little S of Rhayader.
There are of course many variations on it. Have fun - whatever you do
