@Yogurt_The_Cow :
(I assume you play in version 1.6 and since you're in year 5 I assume you have everything unlocked - might not be the case though, plz play at your own pace

)
Braziers and Lamp-Posts:
- Did you actually try them out?
I didnt for a long time.. and was astonished,
they light-up your farm WAY MORE then I thought, and then torches do, f.e.! (Wiki says torches 3 tiles and Braziers and Lamp-Posts approximately 7 tiles)
--> You might need way less of them, then you think and therefore need less materials!
- The difference between them?
Braziers can be toggled on and off, Lamp-Posts are always on + Lamp-Posts provide a higher amount of light.
Considering this and that you
mentioned not wanting to use Iron or coal,
my recommendation for you would be:
--> Wood Lamp-posts. Takes 50 Wood + 1 Batterie to craft. (Buy the recipe at Robin's Carpenter Shop for 500g)
Other recipes without coal are Campfire and Skull Brazier (both mentionned above...as are the Jack-O-Lanterns - but (!!) those provide even less light then a torch, but need a torch to craft (if you don't steal them)+ a pumpkin - selling it you could buy 1-3 coal (depending on quality + if pure or processed..(wine with artisan but 75% should give more or less 756?) + They do go bad(!)... One could craft Marble Brazier (but marble is quiet rare) + the Island Trader provides you with Jungle Torches for 5 Cinder Shards each! (often considered hard to get too, though.)
Coal:
I find coal not to be that big of a problem anymore since 1.6. Buying is expensive, that's true. And they are enough uses for it, sure. But I believe it shouldn't be THAT big a problem as to stopping you from crafting a few Braziers! To
gather consider:
- The
coal-carts in the mines refresh their content at the start of each year
-
Coal nodes!
-->especially in the
quarry! clear it entirely to give more room for new coal nodes!
-->They're in the Quarry Mines and in the Volcano too
- Farm
Dust Sprites in the Ice Floors of the mines (41-79), eventually enhance that using monster musk.. (And use the occasion to gather iron!) (+if no dust sprites and no iron on the floor, don't hit rocks till you find a ladder, just take the elevator!) (+once you've done it enough it's enhanced through the burglar ring)
- Anywhere,
rocks might drop coal.
Chance strongly depends on: luck + the profession you use + can get even higher with the statues power.
(- I usually don't do that but since you said wood isn't a problem, you could use charcoal kilns to convert 10 Wood to 1 Coal. )
- Consider
other sources: Opening Geodes, Fishing Chests, panning, (If there) Clint's shop at the desert festival, Recycling machine, Bookseller (Mining Monthly), Clint's Garbage Can, trains, artifact spots, crates, barrels, monsterdrop (not only dust sprites)...)

Now you could always
stay with torches. Torches on Sprinklers, Fences (as others mentionned above) should provide enough light for the areas with crops and with animals. Maybe some to light up your main walking paths.. could that be enough?
-->
How much light do you need/want? Like..Do you want it fully lit up? Or do you dislike those dark passages between 2 lights? Or do you want to be able to see where you walk? to do farmwork at nights?

Personally, I:
- like to have torches or braziers at both sides of entrances (farm entrance, house, bridges, secret passages etc)
- sometimes have torches on my ways (you can walk on them) or to each side of them
- like to surround my crop area(s) and sometimes light up the sprinklers.
That's usually enough for me. I don't need my
entire farm lit up.

But therefore:
- I always wear a
glowing ring. Later in game it ends up being an iridium band combined with another ring of my choice...
- My
lighting on my displays is turned up -->Then it doesn't get
really dark anymore, I can see way recognize stuff... + That way I find it less disturbing, having some areas lit up and others dark, since the difference isn't as harsh.
--> Maybe that helps you needing less lighting items?