Thanks a lot for your help guys.
I am going to forget about Chinese groupsets and just get 105 or slowly gather used Ultegra/105 to finish a bike in time for summer.I have no idea what a derailleur with a clutch mechanism is, do the shimano road derailleurs have this feature? I'm going to need to do some research. Why does going 1x require a bigger cassette? is it so that you can get a bigger range of gears?
The clutch is effectively a much stiffer spring in the rear derailleur to allow higher chain tension. Its useful for riding over rough terrain as it stops you chain slapping around so much. Shimano 105 does NOT have a clutch, only shimano GRX and MTB derailleurs do. You're better off looking up a youtube vid if you want more detail, will be much more intuitive.
Yes, to get a bigger range of gears, this is a big minefield and was the hardest decision when I built my first bike.
Example; 2x setup with 50-34 Crankser and 11-34 Cassette.
The easiest uphill gear is 34/34
The hardest downhill gear is 50/11
Example2; 1x setup with 40t Crankset and 11-34 Cassette.
The easiest uphill gear is 40/34
The hardest downhill gear is 40/11
In both extremes of the gear range you are more limited than the 2x setup. Therefore you have to compensate with a bigger cassette such as 11-50 which will be heavier but give you an easier uphill gear of 40/50. You are still limited in the hardest gear to the same 40/11 which is why you see most speed focused roadies opt for 2x.