You don't need an airbrush to do decals etc, but it really does help. More on that in a bit.
Decals are the easiest way. Fallout hobbies are I think the go-to for custom decals (which you'll need as
GW don't make those).
For now, you just need to have a bit more patience. Gloss applied by brush will take longer to dry than airbrushed, and it's sometimes unwise to try to speed it up with a hairdryer - if too thick, the air flow will push it and make it look lumpy.
Winsor & newton galleria gloss self levels quite nicely for brush application. I'd give it overnight / 24h if applying by brush just to be safe.
Once gloss is dried, microsol and microset are your friends.
This is my tutorial (which people almost inevitably link, and which covers the main pitfalls):
https://www.winterdyne.co.uk/maz/winterdyne_tutorial3.pdf
Hope this helps.
Back to the airbrush: I strongly think it's an almost essential bit of kit if only for priming and varnishing (which you end up doing 3 times if you want a very durable decal - glosss before, gloss after, and matt at the end) - especially if your access to outdoors is limited. Yes, it's an outlay in cash initially, but it saves SO much time and money that I think you really have to be deliberately set against it to not pick one up.