Your assumption here is that, since sanctions strengthen the regime, not having sanctions weakens the regime, which is not logical.
Not having sanctions potentially strengthens a regime more than sanctions do, embeds them in the global geopolitical/cultural/economic stage, normalises their behaviour, and goes against a lot of people's deontology.
Look at Israel: no sanctions, strong Zio regime, majority of US/German pop supported the "self-defense" argument for decades, complete normalisation of Palestinian genocide until the horror reached an unbearable threshold. Etc., etc.
Yes, sanctions are far from perfect, but I strongly believe that a world with Israel santioned would have been a much better place for everyone, including the Israelis (from having to contend with their ideology).
Edit: I'm also aware that my argument is not perfect either. For example, I wouldn't qualify what Cuba has or what Iraq had as sanctions in the sense that I'm talking about: these are to my eyes an economic war of aggression by the US/West. What I'm defending is sanctions on fascist and ethonationalist global/regional superpowers that are engaging in large-scale horror. But I'm aware how leaky my definition is.