I have to say, wonders giving additional theming boni would be quite much imo. Wonders already are very potent imo. Only the Great Libary is not that good imo, so it would be nice if it would have one. Oxford on the other side is one of the most powerfull wonders and one big help if you want to rush the science victory. Making it even more powerfull for the culture victory would be quite too much imo.
But yeah, theming art museums is really pedestrian. In most cases, you want get many great artists in a row and getting 3 GA who all are making one type of art piece seems very luck luster to me. So you have to deal or steal. Until now, I couldnt really get sth stolen, so I dont know how viable that strategy is. On the other hand, you can make deals with the AI about Great Works, but in the diplomacy trade screen, you just have the names of the GW, sure you can see if it is a writing or a painting, but you have no information about the details. The only way to get those information is, that you have to look at the Great People tab, go to recruted ones and look there for someone needed and then you can hope, that the AI will trade with you.
A simple mouseover information popup in the diplomacy trade screen would hugely help in that matter ...
So yeah, I have to agree, archaeology museums are better, sure you have to invest hammers in the archaeologist, but archaeology sites can spawn inside your empire on unimproved tiles and will block it for any improvement (not sure if it even blocks districts and wonders), so sooner or later you want to get rid of them anyways, because they have no other use. Another downside, you have to wait for the civic longer then the GA, to get your archaeology museum started with works.
And until now, im playing just on emperor, but after I finished my current game, I will move on to immortal, but until now, on King and Emperor at least, it doesnt seem to be that the AI is hunting artifacts like it did in BNW. But on the other hand, the AI can go crazy with the GP. I already have the feeling, that GP are the new wonders in Civ 6. Wonders will be hunted by the AI too, but I just got beaten once by it for an important one (Ruhr Valley), so I was really disappointed. But the AI beat me several times to GM and GE even with Japan having in all my cities CH and IZ and using projects, so I missed James Watt (one of the MOST important ones imo) and those with the unique luxuries.
Same goes for Great Artist/Musician/Writers, if you are unlucky to have Russia ingame, it will need a big investment to get some Great culture people.
So long terms short, yeah, archaeology is better then Art, the Museum even gives the same GPP, only downsides are needed production and it takes longer to get to the point to get your archaeologist working, because of the civic needed for it.