you are using public WiFi from a place far away from home so your ISP doesn't know you will be installing GrapheneOS.
Then the instruction is to not use the phone at home or use a Faraday bag about a mile or two before arriving home.
I don't understand what this mean. What is your threat model?
What is the best approach if I want to use the GOS phone at home?
Depends on your threat model, if you don't care about ISP knowing you are installing grapheneos on your phone, just install it normally, otherwise, use public WiFi or setup VPN on router (advanced). Then install VPN and connect to it with kill switch so no leaks happen and keep Airplane mode always turned on so no IMEI is sent. Don't insert of course a SIM card if you don't want to.