Love these composers from the North, and I've been meaning to reply, but I was not sure who I already owned, and I'm quite interested in obtaining more! So, just goin' through my database & checking agains the Wikipedia lists from these countries, I've generated just a 'small' list of composers, most of whom I already have recordings of their music - two exceptions that I see right off are
Englund & Melartin!
But, already in the pages of this thread is mention of other great sounding composers who I would like to explore - and there are probably more! In fact, CLICK on each country - this will take you to an alphabetized listing of composers from each place; there are 50+ to 70 individuals listed per country -

I assume many of these are not major figures or may even not be composers, but the nembers are long (and most I do not know).
BTW - included
Kraus & Tubin in Sweden because both spent most of their composing years in that country - but I'll keep a close 'eye' on others mentioned -
Denmark Dieterich Buxtehude (ca. 1637–1707)
Louis Glass (1864–1936)
Vagn Holmboe (1909-1996)
Friedrich Kuhlau (1786 – 1832)
Niels Gade (1817–1890)
Carl Nielsen (1865–1931)
Christopher Weyse (1774-1842)
Finland Bernhard Crusell (1775-1838)
Sven Englund (1916–1999)
Erkki Melartin (1875–1937)
Einojuhani Rautavaara (1928-)
Aulis Sallinen (1935-)
Jean Sibelius (1865–1957)
Norway Edvard Grieg (1843–1907)
Agathe Grøndahl (1847–1907)
Christian Sinding (1856–1941)
Rolf Wallin (1957-)
Sweden Kurt Atterberg (1887–1974)
Franz Berwald (1796-1868)
Joseph Martin Kraus (1756-1792; born Germany)
Gustav Allan Pettersson (1911–1980)
Ture Rangström (1884–1947)
Eduard Tubin (1905-1982; born Estonia)