I'm not sure it counts as an answer, but I, like I think many people who are called upon to read them, have found SGA3, and [the œuvre of Bruhat–Tits](https://mathscinet.ams.org/mathscinet/publications-search?query=au%3ABruhat+au%3ATits), both quite daunting.  In both cases, the remedy has been, with encouragement from wiser colleagues (thanks, Brian Conrad, Sean Cotner, and Stephen DeBacker, as well as anyone else I've forgotten!), just to sit down and read them, and find that they are by and large beautifully written.  For SGA3, this is helped immensely by the lovely [réedition by Gille and Polo](https://webusers.imj-prg.fr/~patrick.polo/SGA3/).  For Bruhat and Tits, J.-K. Yu's introduction [Bruhat–Tits theory and buildings](https://doi.org/10.1090/fim/026/02) was a wonderful roadmap.