I only connect to Nintendo sever on ofw
If its the same nand, it doens't make it safer, switching between ofw and cfw is only usefull if you splitt them.
Its because one of the main detection methods of CFW are error logs, which might get written to the nand while in CFW, depending of what sxos adopted from atmosphere. When you then connect in OFW they will get send to nitnendo.
Thats something that even using the stealth mode while in CFW will not change. Error logs already send to nintendo should also never be deleted, since nintendo is at least in theory able to detect this.
Another other method of cfw detection might be the tickets on the console.
Invalid tickets, created by installing homebrew or game nsp installation might be detected by nintendo, when going online, even in OFW, since the tickets themself stay on the nand, even in OFW.
Thats also the reason why most people suggest using the emunand, it will split the sysnand used for online from the cfw nand used for NSPs.
I myself nerver used the emunand, but was always carefull, by using things like atmospheres creport and error module even in sxos, and never installing anything, until I fully moved to Atmosphere, because XCI loading without online support wasn't worth the intransparency and limitations in homebrew and sysmodule compatibility for me anymore.