SCOTUS would say that the distinction is that we don't have a fundamental right under the Constitution to have a swimming pool on our property. But we do have a fundamental right to possess firearms.
As established in District of Columbia v. Heller and McDonald v. City of Chicago, the Second Amendment protects an individual's right to possess firearms for lawful purposes, such as self-defense. Any state law impacting this right would be subject to judicial scrutiny and likely strict scrutiny. Strict scrutiny is applied when a law impacts a fundamental right or involves a suspect classification. Such laws must be narrowly tailored to serve a compelling government interest.
While the right to bear arms is protected, the Supreme Court acknowledges that this right is not absolute and can be subject to regulations. Restrictions such as background checks and prohibitions for certain individuals (like felons or the mentally ill) have been upheld.
However there is legal precedent that excessive economic barriers to exercising a fundamental right can be problematic. For instance, in Harper v. Virginia State Board of Elections (1966), the Court struck down a poll tax as it constituted a barrier to the fundamental right to vote.
Given these principles, a mandatory $300,000 insurance policy could be seen as a substantial economic barrier to exercising the right to bear arms. The Court would likely assess whether the law is justifiable under strict scrutiny. If the state argues that the law serves public safety, the Court would consider whether it's narrowly tailored to that interest.
If the requirement disproportionately affects lower-income individuals, the Court might view it as an undue burden on the fundamental right to bear arms, similar to how poll taxes were viewed as barriers to voting rights.
All of this is very stupid, and does not happen in normal liberal democracies