Bus is not a sensible option. Anyone who lives there uses their own airport, so there is no demand for buses. You have to change at least once.
Reading is not a sensible rail option. It takes the same time to Reading on the bus as it does by train to Euston.
Reading to Manchester is hourly, on very busy short trains which take 3:15.
Euston to Manchester runs 3x an hour, and takes an hour less, on trains with far more capacity.
Flying is ludicrous, the flights have not been "frequent" for about 30 years. There are 5 a day.
The far better rail service collapsed demand. By the time you have waited for a connection, flown and transferred to train you have arrived by train direct from Euston, probably hours earlier
If you wanted to avoid London you would take the twice an hour rail air bus to Watford Junction, frequent train to Milton Keynes Central, then hourly train to Manchester. But somewhat slower than via Euston, still better than via Reading.
There was a time, when Manchester was a single terminal airport, when flights to Heathrow were almost like a bus service, they were so frequent. There was a hot spare aircraft at either end in case either of breakdown or excess demand for what used to be about hourly flights. So you could turn up on spec with a flexible ticket and have a guaranteed seat.