The strongest boundary is the one that does not require a speech.

A boundary that has to be explained, justified, and defended tends to invite negotiation, because the explanation implies there is a case to be made and therefore a case to be argued against. The most durable ones are short and simply held: no, not this, not available. Their strength comes from consistency rather than from the reasoning behind them. You are allowed to have limits you do not defend.

Practise a one-sentence version of a boundary you keep over-explaining. Say it plainly and then stop talking, which is the hard part. Repetition without elaboration is what actually establishes it.