need advice on which lines to grind

Hello, noticed my account is pretty weird consisting mainly of premium tanks and not nearly enough tech tree tanks like I would want it to be.

right now my tech tree looks like (completed lines)

Kranvagn, M-V-Y, Vz55, M4 54 (just bought it today, stock gun as of now), also have a fully researched UDES03 that I plan to finish grinding somewhere in the near future – absolutely love it

lines I stopped grinding : E5 (at T32 maxed), E100 (Tiger II), Rinoceronte(the tier7)

be aware I am mainly a medium tanks player, with like 20-25% on heavies.

just started grinding s.conq line (which I heard is really good)

I always grind two lines at a time in order to maximize the time.

which lines should I grind with it or next after?

was thinking about UDES 15/16 maybe ? even though I dont have a swedish prem medium

overall my most preferred options :

STB-1 (immense tank – played it on a friend's account – beast.) , UDES 15-16, TVP 50/51

Would love to hear your thoughts.

P.S ; I know the lansen c is pretty good to pair with udes, skoda t27 is horrible (played it too), how's the STA-2 to buy if I start grinding the STB?

EDIT : Is the T57 Heavy still relevant in 2023 to research ?

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