For budget entry eyepieces, the generic "goldline" eyepieces have pretty good bang for their buck, especially the 9mm/6mm ones. You can find them unbranded online, or the "redline" equivalents.

I'd recommend a quality 20/25mm plossl rather than the gold/red line 15/20mm that display a lot of issues.

For better eyepieces, at f/5, the rubber-top generic eyepieces with color bands sometimes branded as "artesky" are great for their prices too.

If you have a bit more budget, Explore Scientific is generally the overall best quality/price ratio. Avoid baader hyperions, except for the 13mm which hits above its price (second-hand)

6" f/5 kits sold by GSO are okay, but the best choice is to find any second-hand skywatcher 6" f/5 tube on your leboncoin/craigslist equivalent and scavenge the optics. For industrial optics, they are on average exceptionnally smooth and have a good shape with few defects.

The 6" f/5 optics kit are a bit of an oddity, those we test with our interferometers are consistently good, whereas smaller or bigger diameters are subject to a lot more vatiations.

If buying a used skywatcher dobson is the best way to get good optics, wouldn't I, as a beginner, be better off just using that dobson instead of going the DIY route? Is the DIY telescope only better for portability, or are there other advantages to building it?

The 6" f/5 can often be found cheap and without a mount, but if you are beginning, absolutely find some secondhand dobson before going DIY.

Or better : attend a few nights at a nearby astronomy club, that will allow you to get a feel of the practice, what you can see, what you should expect...

Depending on your location (city? middle of nowhere?) and willingness to drive to darker skies some types of instruments and targets are better than others.

This kind of kit is focused on being compact and lightweight, but that can be an advantage or a non-goal depending on what you want to do.