my suggestion would be to use valheim plus and enable the option for fire/torches if you only want lighting to be always filled and not fire places thats an option too.... here is the config section in question from the valheim_plus.cfg
[FireSource]
; Change false to true to enable this section. https://valheim.plus/documentation/list#FireSource
enabled = true
; If set to true, torch-type fire sources will stay at max fuel level once filled.
; Applies to: wood torches, iron torches, green torches, sconces and brazier.
torches = true
; If set to true, non torch-type fire sources will stay at max fuel level once filled.
fires = true