there is some ktard in kopitiam that builds custom pc as that is his actual day job
sometimes he will post pics of him building it maybe you can ask him
but if you ask for custom build and want the store to build it for you, the price may be the same as buying pre-built since the store will charge a fixing fee to fix it up for you which may or may not be cheap i.e. 10% fixing fee of your entire rig so if you buy 6k pc then its 600$ fixing fee
there are lots of ktards that love to build it for free for you but you'd be gambling on which one is sane and which one is insane
honestly building it your own is very satisfying but I can understand if you dont want to coz cable management is a pain in the ass and its actually a lot harder than what people say
so what you should do it compare the prices of custom + fixing fee and prebuilt
also, for 5k you can get the top 30% of the line specs. if top up 1k to 6k total you can get top 10% of the line
but you need to give more details on what you want to do with the pc before people can suggest something legit.
the one advice I'd give is dont cheap out on psu. I see lots of people suggesting no name brand psu but if you're gonna be powering a rtx 4090, im sure you want a reputable psu instead of a house fire right?