How would you paint detailed golden wall decorations?

To be honest with you, I wouldn't attempt it!  The work of art - the decoration and gilding - has already been accomplished: the best I could do would be to feebly imitate it. IF I were to try, I would go for the illusion of detail - the impression that such decoration immediately gives rather than a detailed rendering.  I do not have the patience for the latter: it would require a great deal of work, and at best all I would have to show for it is likely to be merely laborious. If you can find him, there is a Polish painter named Paul Dmoch, who paints, among other things, church and ornate interiors in watercolour, suggesting the detail while still conveying the majesty of the setting - I'm not aware of many oil painters who have achieved the same, but I shall Google 'oil paintings of elaborate interiors', or something similarly worded: it would be interesting to find such artists.  I have no objection to detail - e.g. in landscape - but this very formal, precise detail - one would have to be nearly as dedicated as the craftsmen who created this profusion of gilding and carving to represent it faithfully. 

Look at the August 2022 edition of Leisure Painter magazine ( which should be available through your subscription) and there is an article by Paul Minter called "Grand Interiors" that you might find interesting as it discusses his technique for painting the inside of historic houses.