There is no demonstrable direct link between high taxation levels and corruption. There are plenty of countries/governments with very low or imperceptible levels of corruption that have relatively high levels of taxation, while there are others with extremely little taxation that are thoroughly corrupt.
Rather, its a question of how and what those funds are spent on. Generally speaking, when you pay your civil servants a living decent wage, and have controls in place, you can drive down corruption over time. When you underpay and fail to supervise people, the temptation rises. Thus, if you don’t tax people, you can get away with a small, but well paid government, albeit a weak one. But if you don’t pay your civil servants, perhaps as a function of poor tax collection, then you’ll definitely end up with corruption, regardless of size or scope of government.
The Greek issue with taxes is largely an issue of a wide ranging historical and cultural elements that does normalize tax evasion, partly as a function of high rates. See here for a brief summary: https://en.wikipedia.org/wiki/Corruption_in_Greece#Extent_of_Greek_tax_evasion
This is also one of many reasons that Greece’s economy is the way it is.
RE: Latvia. Has about 21.6% estimated tax evasion, but also has a rank of 40 in the Corruption Perception Index: https://www.transparency.org/news/feature/corruption_perceptions_index_2017
Greece is 59 in that listing.
Greece has fairly high taxes. That does likely feed into tax evasion. But solving tax evasion is a long running issue, and lowing taxes year over year won’t necessarily suddenly convince that portion of the Greek populace who have serially cheated on their taxes to start paying their taxes.
