These are some tips for progressing in the game.
The guide isn't complete, and there might be some sub-optimal decisions.
But I've reached adventure level 3000 and enjoyed it, took 240 days of AFK on my home server. And since there are literally no guides about it now, I shared my insights.

Some things to note, so we'll be on the same page:

• The guide describes playing this productivity tool as an idle game, that is 24/7 running timer. Some might say that's not the point of this game. I somewhat agree, but that doesn't mean you can't have some additional fun running it on a separate PC.
  
• The guide describes the lazy AFK approach to the game, so you don't need to check it too often. Once a day is enough. That said, I'm sure there's a quicker and more optimal way to do things, but personally, I value my personal time and energy.
  
• My game settings are "Vegetarian: on" and "Enable fish trading: off". It affects the names of some items. And I don't want to know how they're called in other modes. But there are only two items which will be really important, so it shouldn't be a problem.

In terms of adventure progress, the game starts very slow, but then it speeds up, if you know what to upgrade first.

At the start of the adventure, buy Tomato, it gives a good return for the investment at this point. Spend only a small fraction on everything else.
If you can afford combat efficiency upgrades, buy only first level. It'll give you a good speed boost for unlocking more items. Don't bother upgrading them after level 1 yet, as it's the thing for a much later.

There are two shop items that are most important: Cookie and Popcorn. You'll unlock them early, and will keep updating them for the rest of the game.
Your spending strategy should always be around these two. Especially Popcorn. They cost a lot of gold, but they'll give you like 99% of your total tomato income, so as soon as you upgrade it, you immediately go to a new level of income.

So when you'll have Tomato at level ~40, start spending all your tomatoes on +gold upgrades and shop items. Your goal right now is to hoard enough gold to buy Cookie lvl.2 and Popcorn lvl.2. When you won't have any gold items to buy, you can upgrade Tomato first, and other upgrades which cost tomatoes, but spend no gold.
This is the slowest part of the game. It'll take a month to buy Popcorn lvl-2, but it'll be worth it. I thought about the better way, but I don't think there's one. But after this, your game will speed up, and other upgrades of these items will be much faster, including the zone clearing speed. Just keep idling and check occasionally for +gold shop items and upgrades.

After you'll get Cookie and Popcorn to lvl-2, there will be more freedom on what to buy. I'm not a big fan of micromanagement, so I found out the general strategy:
Initial setup:

• Order shop and upgrades by cost.
  
• Use macro to click on the cheapest item for 24/7.

After that, there are 3 sub-strategies which you'll swap from time to time:

• Upgrading items
  
• Pushing adventure progress
  
• Saving for next Popcorn

Popcorn part is the most important one, but it's not always possible to buy one in a reasonable time without switching to other strats.
Saving for next Popcorn: Click on your gold, it'll show your earning rate. Use a calculator to measure the time needed to buy the next Popcorn. If it's a couple of days - it's fine (it's a long game, though it'll be faster on later levels), if a week or more - not so good, you need to increase gold earning, so to other strategies...

Upgrading items: buy everything for +gold. New combat upgrades to lvl 1. You can move to the next phase right away, or, if there's still a lot of upgrades possible every finished pomodoro cycle, set the before mentioned macro to buy other stuff maybe for a day or two. Not the most efficient way, but I found it a good balance of progression and needed attention. If you're bored at work, this is the place to procrastinate and do more something manually to get your Popcorn faster.

Pushing adventure progress: When there's no gold shop upgrades left to buy, the only way to progress is to push faster through adventure to unlock more upgrades. And the fastest way to progress at this point is buying all shop items (temporary buffs). The AFKy way is again to use a macro for clicking to buying cheapest "potions" for 24/7, remember to sort by cost, so you won't need a smart macro, but just clicking on same spot. Check the game daily to see if there are new unlocks in the shop. Spend everything in the upgrades once in a while, because that's how shop items work: after a couple of hours of buying them, the effect snowballs and you'll start getting a big bonus. But I propose stopping this snowball sometimes to wait and get the big upgrades earlier.

After you get some new +gold upgrades, try to calculate if time to get Popcorn is now better. If so, switch to gold hoarding mode again, and only buy gold upgrades and shop items.

Crystals are prestige currency you get for retirement (restart), and it can buy powerful upgrades.

It can also be obtained from daily tasks. So in this "once a day" strategy I'm proposing in this guide, don't forget to collect the rewards. 16% of my crystals I've got from dailies, and you don't need to wait for retirement to use them.

As for what to spend them on, I think the best bang for the buck is % for combat efficiency, starting from the most recent you've unlocked.

Constant + to combat efficiency give very good increase at first level up, but after lvl 1, percent upgrades are more efficient.

Cost reduction updates don't give enough for me to vote for it. Too small %, and combat efficiency kinda has it included, because you'll unlock more shop items and upgrades which will instantly boost your earnings.

But buy every crystal upgrade at least once, as they all give a good initial bonus.

I would say go straight to the last adventure zone, for 2 reasons:
1. The latest crystal upgrades you'll unlock by progressing through zones are the best. Kiwi and Avocado. You'll get more bang for the bang when you upgrade them after retiring.
2. The game doesn't slow down to a crawl anywhere to force you to prestige. (And it's a good thing, thx dev, I can't stand other so-called "idlers" with bs mechanics).

That's it for now, maybe I'll add or correct something when I'll know more.
I hope it was useful and you'll have more fun in this chilling game :)