How did you learn to do boundaries/overcome FOG? Side effects?

[qcdlvl]

I was lucky enough to learn them as a child - I didn't know what they were called or anything like that, but I learned from seeing others enforce them and in a way having toxic, likely PD people in my extended FOO was a blessing in disguise.
The first Christmas I remember very distinctly, there had been a falling out with a relative. Now, my GM was very much of the "blood is thicker than water" school and it was particularly important to her to have Christmas with all of her descendants, or as many as possible, under the same roof. So she tried to force a "reconciliation" - so she announced she was spending Christmas with this relative, rather than at a more neutral venue like her place (I think previous Christmases had been at her place). So my parents simply responded that we'd spend it at our place - she was very angry about it but there was nothing she or anyone else could do about it.
There was later an incident with a toxic, very likely PD relative that, in a different jurisdiction, would've probably led to charges or been grounds for a lawsuit against this relative.  My nuclear FOO went NC with this relative for years, despite FOG horns to reconcile, holiday drama (is it any wonder I'm no fan of Christmas?), etc.
There also a sort of family feud related to money - my parents' stance was something like "We're prepared to pay $X (essentially, the appraisal of an independent professional, not some number pulled out of thin air) and if it's not acceptable to you, then the deal is off" - there was a lot of drama,  FOG horns, stalling and attempted trickery, but eventually the other parties agreed to it - it seems the price was right after all, as they could've simply called off the deal).
So these and other incidents taught me a lot about dealing with "difficult" relatives - that blood is thicker than water, but so is b******t, that you can say no, set boundaries, cut off contact, grey rock, etc. Perhaps they also made me a bit paranoid  - after all, in a way, my immediate FOO was under siege for much of my childhood and teenage years by others who wanted rugsweeping of abusive, vicious behavior, money, to dictate holidays, etc. So now with my FOC I feel hypervigilant - perhaps overthinking what I might need to guard against, even though in practice my toxic relatives (FOO and ILs) have no real leverage and hopefully little interest.