Complex sentence: Ford Madox Brown was invited to join, but the more senior artist remained independent but supported the group throughout the PRB period of Pre-Raphaelitism and contributed to "The Germ".
Split into simpler sentences: Ford Madox Brown was invited to join, but as a more senior artist, preferred to remain independent. He nevertheless remained supportive of the group throughout the PRB period of Pre-Raphaelitism, contributing to "The Germ".