You may have read some of these:
List of Slavic mythological figures
SLAVIC MYTHOLOGY The Gods of Eastern Europe and Russia…
Polish Paganism (possible UPG and neo-pagan/feminist stance, still might be good to get ideas for further research)
Polish gods part 1 – historical and seems interesting
SLAVIC and EURASIAN PANTHEONS – “This page is intended as a reference guide for students of Slavic and Eurasian mythology, and is a catalogue, hopefully reasonably complete, of known God-forms.”
Reakcja pogańska
“Reakcja pogańska – termin współczesny, którym określa się serie wystąpień ludności ziem polskich okresu wczesnofeudalnego, skierowanych przeciwko wzrostowi powinności prawa książęcego oraz przeciwko odgórnemu wprowadzaniu chrześcijaństwa. Punktem kulminacyjnym tych zaburzeń było powstanie ludowe, które wybuchło w 1038 roku, w Wielkopolsce, doprowadzając do anarchii wewnętrznej.”
From what I found, the lack of written records will mean you need to contact your gods directly, and rebuild your own system from there – no bad thing IMO if you have the desire to do it!
Much Norse neo-paganism is muddled with new age concepts that are outright incorrect, and the true brutality of our gods and the people they mentored and walked with is buried under feel-good assumptions, that work well in our tightly-crowded, and reasonably comfortable, modern urban lives, but made little sense back in the day…