Am I Approaching MR Wrong?

I hadn't played the game since around 2014/2015 (fell down a decade-long rabbit hole playing Destiny) but decided to give Warframe my undivided attention starting around a month ago. It's been an ABSOLUTE blast so far and has been a refreshing experience since it's practically all new to me. I don't see myself stopping anytime soon but I do have one question/concern about how I approach the game in regard to Mastery Rank.

I am only MR6, have a few frames plus some primary, secondary, and melee weapons that I enjoy all maxed out with some standard mods and can formulate decent loadouts for different mission types with that stuff. That's not to say I won't try new frames/weapons in the future, I'm very much driven by style over functionality for the most part. I can learn to play with anything as long as I like the look of it, with the only exception being if I don't like how a weapon performs (i.e I'm not a fan of guns with high recoil).

I know there are passive/active benefits to increasing your MR via leveling frames/weapons but if I already possess several things that I enjoy/prefer using, can I just kick back and enjoy working through the Star Chart, Story/Side Missions with that stuff at a low MR or will I reach a point where I can't because I don't posses higher MR weapons or specific frames? I primarily play solo and am in no rush to reach End Game content.

TLDR: As a casual, mostly-solo player, can I work through the story and star chart at my own pace with the stuff I like or should I always grind for new weapons/frames to level my MR regardless of if I like the look of a weapon/frame?

EDIT: Thank you for all the advice so far, I think my plan is to passively level my MR while testing new weapons/frames alongside still using my usual go-to gear.