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.
You can do sanctions on items that allow the regime wage wars (weapons and dual-use products), yes, that can work. Or wide sanctions on small countries such as Israel can be a credible deterrent, since it lacks economic depth to find substitutes.
However, wide sanctions on large countries such as Russia or Iran are now proven to be quite ineffective in the long run. Even worse, by preventing the creation of a middle-class, you won't have the conditions to start a democracy later, after a possible regime change.
I know it sounds counter-intuitive, but it's what data shows.
And sanctions don't prevent countries from committing atrocities either. What about the deaths and suffering induced by sanctions? 500k Iraqi children were estimated to have died due to the US sanctions. The architect of the policy told that it was "worth it". Was it?
https://www.newsweek.com/watch-madeleine-albright-saying-ira...