I don’t think that you qualify for the Beckham Law as your employer will not be a Spanish company or entity (please confirm).
Keeping a home in Belgium hurts you regardless of what you decide to do. Having Spain or Portugal as your registered home or tax base also brings no joy. You have an EU passport and will not loose it by moving roots. Consider selling the home in Belgium and cutting all ties with Belgium, Portugal and Spain. Open a company in Tbilisi Georgia and have your German employer contract with you via your new company.
Georgia is a territorial country, so will not tax earning from outside. Your passport gives you 12 months to live and work there without a visa. Starting a company there helps get you residency and can lead to getting their passport.
You don’t have to live in Georgia. Remember that as a nomad, you are free to go where pleases your heart and wallet. Have a look at Serbia (Belgrade for full effects and Novi Sad for a chilled lifestyle), Malaysia, Montenegro, Chiang Mai, Ho Chin Ming and Armenia. For less than 900 euros a month, you can live very comfortably with some accommodation having a pool, gym, internet and cleaning service included in your rent. From January 2020, Serbia will be giving visas to digital nomads to come, live and work tax free. A studio in the city center of Novi Sad can be as low as 130 euros per month. Any home you buy in Serbia gets you residence permit. Your annual tax will buy a flat in the city of Belgrade. For 6,000 euros you can get a house on an acre of land about a hour from Novi Sad. Primary school is free and compulsory, Secondary school is free but not mandatory in Serbia.
Regardless of what you choose, your tax burden in Belgium, Portugal and Spain would be the same or more than the cost of living in these other locations. Your entire earnings and assets are now free and available to you to invest in low/no tax passive investments. You can still visit Belgium, Spain and Portugal anytime you want (but have to keep it under 183 days a year). Get an international health insurance cover and travel insurance.
I will be the first to admit that cutting ties with a country you know and love will be hard. Also, some/all of theses locations are not to everyone’s liking. What I am saying is that you consider these types of options that give you the chance to keep more of what you earn, save more, invest more, spend less but live a full fulfilled life with less burden.