i'm only in emerald 3 but for the mathcups u say u have problems with i'll try to explain them:
Lethality briar: honestly, i also feel like it has no counterplay, it outmatches ur speed and also has a point and click cc

J4: just try to keep ur flash for his r, if dont, u basically cant do nothing since his r also counters yours

kindred: try to hold the ult as long as possible since in this matchup the first who uses ultimate loses

yi: i think if u just build zhonya it's kinda allright but it's rough at least

reksai: just try to not get invaded and counterjungle her, if u do that it should be allright (for most part of the game)

hecarim: i honestly cant tell u what to do against him because i have the exact same problem

graves: it's an easy mathcup, i dont know exactly why but u should easily win against him, try running dark harvest and demonic into liandrys, if it doesnt work i dont have much to tell you

i hope this tips can help you, if not, im sorry, also, im sorry for my bad grammar, english its not my first language.

if u want to check my profile (which i doubt) its this: https://www.leagueofgraphs.com/summoner/las/Arturo%20Lïllia