While web search results confirm Bulgaria has faced anti-corruption protests and political instability, the claim specifies the crisis started in 2021 and names Boyko Borrissov as the leader whose government was toppled. The evidence confirms anti-corruption protests and instability since 2020/2021, but the specific narrative of the 'conservative government of Boyko Borrissov' being toppled *since* 2021 is not consistently corroborated across independent sources, although the general context of anti-corruption protests is present.

The cross-reference source explicitly states Radev advocated for renewing ties with Russia and slammed military aid to Ukraine. However, the web search results provide general context about his views (e.g., Eurosceptic, pro-Kremlin) but do not independently corroborate the specific combination of 'advocated for renewing ties with Russia' AND 'criticized military aid to Ukraine' from multiple independent sources.

The web search results mention Radev's geopolitical stance and his opposition to EU policies, but none of the provided evidence explicitly state that he 'officially denounced Russia's invasion' AND 'stated he would not use Bulgaria's veto to block EU aid to Kyiv if elected.' This specific combination of statements appears to originate from a single context or source not fully captured by the evidence provided.

Wikipedia sources confirm elections occurred in 2023 and 2024, establishing a pattern of elections. However, the specific poll data suggesting Borissov's GERB party is likely to come second with around 20% support, ahead of PP-DB, is not corroborated by the provided web search results, which focus on Radev's polling numbers.