There are two achievements directly relating to workshop content:

Unfortunately the Workshop integration is somewhat broken. If you go to Patterna's Community Hub, you can still access the Workshop tab in Steam, or you can click on this link to the Workshop. From the top menu select "Browse" and click on "Puzzles". From there you can subscribe to individual puzzles, which will then show up in your in-game level menu. At the time of writing there are 7 puzzles available, all of which are very easy.

Patterna has 26 hidden achievements:

There are only a few bottlenecks, which largely dicate your gameplay. Rank does not matter, because it is very easy to grind for level 25. You will likely reach this rank long before 50k nodes.

How much time you will have to spend on grinding random levels will largely depend on the settings you choose. I prefer the "Full Hex Grids" level generator, as it never produces directed edges and has a very easily understood layout.

The first big bottleneck is "Reveal 10000 nodes in hardcore mode". Enable hardcore mode, set size to 100 and grind away. Choose the difficulty according to your preferences - 0 is boring but fast, 450 is a nice middle-of-the-road setting I very much enjoyed, 999 is slow but challenging. It will take several hundred levels to complete the hardcore grind. On the way you'll have completed most node and random level achievements as a by-product.

Of the few achievements remaining the next big bottleneck will be "Finish 1000 randomly generated levels". This is a tall order, as so far you'll probably still be under half-way there. First - disable hardcore mode. No reason in failing levels or scoring higher, when you're just trying to maximize the number of levels completed. If you still enjoy the challenge, by all means, play on larger maps. The optimal play is going down to size 0 though, as it is not time-effective to grind nodes in random mode.

By now the only achievements left should be revealing 50k, 100k and 500k nodes. The quickest way by far to grind nodes is setting up a custom level with 120 non-pattern nodes. Unfortunately there is a hard-cap of 120 nodes per level. Even if you manually edit the level files, the game just crashes loading any level with more than 120 nodes.
For your convenience I have set up such a level and uploaded it to the workshop - available under this link. The loop is as follows: Select the level from the menu, hit play, wait for the level to load, hit the "solve everything" button in the lower left corner. As soon as the score screen appears you hit Esc, bringing you back to the level selection menu, where you can hit play again. This cuts out a level transition animation (which are the slowest part of the whole grind).
Sidenote: For whatever reason my custom level adds 240 revealed nodes whenever I play it, not just the 120 it actually contains. I think this is a general error and not specific to my level, but either way it greatly speeds up progress (it took me around 35min to grind 20k nodes).
Sidenote 2: There appears to be slightly different tallying of nodes between ingame statistics and Steam (according to the developer). The 50k achievement only popped at 50,820 for me, the 500k achievement I got at ~497k. Editing your local statistics file does not affect Steam's count and thus the achievement progress, so I'm pretty sure you don't need to keep a consistent save file if you add to your node count across several years.

I played fairly straight-forward with large sizes and moderate difficulties for a long time and only started employing grind tactis in the last few hours (which is when most of the progress happened), but it still took me around 65 rather casual hours to get every achievement except 500k nodes. Personally I don't think it's really worth a few years of casual play or 9-10 hours of mindless node grind for the last achievement (I did it with a very small window size while I was watching videos in the background), but that decision is of course up to you.