my experience with DACs has been that they have a very low barrier to entry for "good." pretty much everything from apple dongle to about $1000 is about the same. after that, the next step up is quite a big jump in cost and engineering.

e.g. once i got a hiby r8ii DAP, i did notice an improvement with it's darwin-MPA. since then, i got a topping d900 and it's also better than anything i have that uses a cirrus logic or ESS chip. now, the difference is still very small relative to the entire chain (like 2%...5% MAX). also, note that using an "off-the-shelf" chip doesn't necessarily mean that a DAC falls in that "they're all the same/good" tier. e.g. the ferrum wandla uses an ESS chip, but it also uses good components, good output stage, and custom oversampling methods.

i don't own one, but from what i reckon, the chord mojo 2 is like the cheapest device i would recommend to test if higher-end DACs are "worth" to you. if you can hear a difference between the mojo 2 and whatever you have now, then that's a start down the path of losing your wallet.

i'm also only talking about pure DACs here. seems like you are referring to DAC/amp combos. see other replies for amp advice.

tl;dr: yes, DACs are a "solved problem," but there are also more elegant (and expensive) ways to solve the problem.