I've been also wondering if there's any good books that just teach all the romance languages at once. For me I think that learning method would work pretty well.

Oo I just googled books for learning all the romance languages at once, a lot of them leave out Romanian cuz it's less fashionable but here's one that includes it!

https://www.amazon.com/Elements-Comparative-Grammar-Romance-Languages/dp/B08FKD7LGN/ref=pd_sbs_sccl_7/131-1618064-0060204?pd_rd_w=bjsrW&pf_rd_p=4b6b5072-e9bd-4f30-a3af-a1f5d52978ec&pf_rd_r=FQV0BPFPAX522QYR77TX&pd_rd_r=f622eda2-3895-4ce7-9704-3012b16c2ce4&pd_rd_wg=ywaaS&pd_rd_i=B08FKD7LGN&psc=1

Now I don't support buying things from Amazon but maybe you can find it through different sources, or other books in a different main language

edit: and the author is Portuguese so that probably helps! his profile says he loves to hear from readers so maybe he'll help you with your languages :)