I can't say I've ever used a roller for finishes, but I'd probably hazard a guess to say it's not the best way.

Personally, I've used a lint-free cloth or foam brush for applying a polyurethane finish, and regular brushes for stain.

Those vertical strips look to be the actual grain of the wood though. What grit did you finish on when you sanded? If sanding isn't the cause, then what about applying a wood conditioner first?

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?

Any time I've had streaks while staining it was from putting too much on at a time, trying to hurry and get it done. Try using a foam roller with a brush or rag.

I’ve never been happy with those stain-and-varnish-in-one finishes. They are the most difficult to deal with. The best use I’ve found for them is for blending in runs of already finished moulding when you join pieces end to end.

Vertical streaks i see are the wood grain absorbing at diff speeds. You just need a few coats of varnish to even it out. Then sand smooth and do another varnish coat or two then polish.

Wood conditioner should be put down before stain, to help even out the stain's penetration.

If it were me, I'd sand off the progress you've made so far and reapply. If you don't care about the finish, then don't bother, but it's something that would bother me if it were my own project.

I'd start with an 80 grit, bring it down to bare wood, then work my way up to at least a 220 if it's something my skin would be touching on the regular. Then, stain it the colour you want with a regular stain, finish it off with a spar varnish (if you're going to have any liquids on the desk, like your morning coffee), to prevent water damage in the future.

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 :)