Given that you have GERD, I am inclined to recommend alkaline supplements like a general mineral supplement for better mental clarity (especially if you don’t eat many fresh vegetables) and magnesium regardless of your personal diet—very few people eating first world diets get enough of it nowadays. Magnesium is wonderful for helping you sleep.

Otherwise, fish oil and psyllium husk fiber are classics and easy to get. A probiotic of some kind is also a must. If you aren’t taking a multivitamin already, I’ve had some limited success with Optimen’s Multivitamin, but it may or may not work for you.

Generally, though, ensuring that your diet is protein and produce heavy will help your brain more than anything. If possible, I also recommend switching to green tea (matcha is the best) if possible instead of coffee, might help with your GERD. If you can’t get rid of it entirely, switching to decaf coffee with a tablespoon of coconut oil mixed in per serving might help.

I tried melatonin once and didn’t find it worked for me; vitamin D was much more effective. But everyone is different.

If you’re willing to look further afield then piracetam combined with alpha-GPC is an absolute must for real enhancement, but sourcing quality piracetam is rather tricky at the moment and may require more money, patience, and research than you can afford. Still, if you can get your hands on some pharmaceutical grade pira, I can guarantee you that the results will be worth the effort.

Of course, if you are having serious issues with regular mental performance it might be worth a trip to the psychiatrist. I knew I had ADHD for years and when I finally got medicated a lot of my life became easier. Just something to consider. My ADHD meds are far from a panacea but give me a baseline that makes any obstacles I do encounter easier to overcome.

Finally, you might not want to hear this, but nothing on earth will ever beat the combo of quality sleep, vigorous exercise, daily sunlight out in a natural environment, clean+fresh air, food, and water, and positive+meaningful social encounters. Everything else I mentioned above simply builds on those foundations; they can never fully replace the lack of any, but most of us in the modern world have to make compromises in one area or more simply to afford to live. It may seem basic but they really do enhance mental performance.

(Would you mind linking the less wrong post you read in your description? I’d be curious to see what they had to say.)

look into cerebrolysin, its probably the best researched nootropic for cognitive function. Sadly it cant be taken orally which is a deal breaker for many. I actually just did a small write up on HYMS' cognitive performance bounty.

Nicotine gums are a good choice. Depends on the country but can often just be bough at a pharmacy.

I can greatly recommend H2O and glucose

Cold shower if you feel tired in the morning

Modafinil is a good choice, given effect profile and safety. If you have problems with the acid in coffee, you might try mate instead, or even just straight caffeine pills.

Adrafinil/Modafinil are standbys for a reason. Also, theacrine, which is best summarized as a cousin of caffeine.