I’ve got one & had the same problem until I learnt how to light a fire properly (never had a fire before)
Fill the burner up to the air holes / Stovax sign with kindling & newspaper layers, couple of fire lighters. Light the fire lighters & close the door leaving about a 1cm gap. It shouldn’t take long to really get going, once it is add some smaller logs & fully close the door with the vents fully open. You need to build up a good layer of embers for the wood to burn on & then you can add bigger logs & start closing the vents to slow the burn down.
Glass never soots up now (unless the in-laws try to light it!), just needs wiping off with scrunched up newspaper.
I also used gel oven cleaner to clean the glass…