I'll definitely give your suggestion another go on the other side. This is just an off-cut of the timber I am using hence the carelessness, so I'll treat this attempt like the real deal. I appreciate your help :)

Thanks for your insight! Might just go down the brush route. Any recommendations for a 1.9metre bit of timber (if size matters lol)?

The vertical strips are definitely the grain of the wood. The grit I used was too harsh for the timber (180). I didn't have any other sandpaper on hand at the time and I didn't care about the finish too much, I just wanted to ensure the colour was the one I wanted. But yeah I'm not sure if sanding caused the horizontal roller lines (I hope not), especially since the RHS was the second coat of stain (applied ~24h+ later). I used 240 grit sandpaper to lightly sand the first coat before applying the second. Would a wood conditioner make a difference for the second coat of stain?