My favourite list of easy hs countries (at present)
Belgium. Many homeschooling coops, activities, activities mainly in Flemish, German or French. Easy administration. Normally no testing or inspection required, depends on the district. Compulsory vaxing (polio).
Netherlands: Not so many hs activities yet and then mainly in Dutch, but it's a small country and you can travel easily. No testing required. You need to apply for an exemption which is easy in these days except if your dc ever (just one day) attended a school, then it's more complicated.
Austria. Ease admin, just sending in a letter of intent. Many activities, but mainly in German. There is, however, yearly compulsory testing at a school of your choice (but it's very easy, first test at about age 7 based on performance and portfolio) or being formally enrolled in a formal accredited distance education program, some US programms are accepted too!
Danmark. Easy admin. Not so common, because parents can easily found their own schools and get $$$ for it. You are required to provide Danish language instruction (you decide how and how much), but that's normally pretty easy.
France. Easy admin if you are enrolled at one of the state accredited distance schools. Not sure about unschooling etc. French is required, but if you are only temporarily in France, you could apply for an exemption for your expat motherlanguage distance school. Many and strong homeschooling groups, most of them in French.
Italy, uncommon, but admin can be very easy in certain districts, just a letter of intent, depends how you present your case. There is little information about hs in Italy.
Poland is often mentioned for easy homeschooling, but I could not find an English information site about that so far.
There are several websites about homeschooling (or home education) in Europe, but some of them are getting a bit outdated, generally it seems to get easier in Belgium, Netherlands and Austria. Ireland and UK very easy (although UK might change). Remains easy in the Nordic countries except Sweden.