I'm so used to playing only two civilizations, worried I will tank ELO if I switch?

My highest ELO is 806 (discounting beginning), So here are my RM Team stats as follows:

Magyars: 251 games, 130 wins (51.8)
Tatars: 312 games, 144 wins (46.2)
Bulgarians (my first civ ever): 76 games, 31 wins
Mongols: 40 games, 18 wins.

There are others that where randomly selected, amounting to 15 total games but not really significant. I tend to play more cavalry or cav arch focused games. I don't think I'm good at this game because for some reason I struggle to get above 800 ELO despite 957 matches (team and 1v1s)

For some reason I seem to really do well in Nomad matchups (62.1% wins) but I struggle with black forest and Arena maps unless I sneak a villager in black forest.

Either way, some of the other civilizations seem so alien to play and I wonder if it will knock my ELO down significantly. I don't know how to play infantry/archer/artillery/elephant style but I've seen them used quite effectively.

How does one get out of their comfort zone? What's a good pick for someone who is used to Tatars and Magyars?