OK, situation about tech speed:
1. Size matters too much
Small civs just can't do anything unless scripted. The most miserable one is Ethiopian, bad city placement and few food and commerce. On contrast, tech speed go skyrocket once you grow large.
Possible solution:
A. Increase commerce from palace, i.e. from 8 to 24, so small civs would have some basic ability. And/or add 1 free specialist for palace, that's reasonable.
B. Increase city maintain cost or stability to balance large empires.
C. Adjust/nerf all buildings, i.e. library from +25% to +15% and 3 raw science beakers. Advanced buildings like university adds maintenance cost.
2. Tech leader gets no match
Once you become tech leader there would be none to stop you. If you are behind, you can steal tech from AI, but AI doesn't know to use spies well.
Solution:
A. Tech-diffusion mechanic from Realism Invictus mod: +25% research speed for each open border civs that already have that tech (50% initial). i.e. Greeks will research alphabet 50% faster after OB with Phoenecians, 75% after OB with Phoenecians and Romans.
B. Teach AI to use spies.
C. Add a visible spy unit: ambassador, auto-generate in rival city upon an embassy establishes. Cannot move without OB, but can still stay in city without OB. This can make all players easier to use their EP.
3. Unreasonable relation between tech cost and date
We all know this, but this is too hard to balance. I don't suggest RFCE method, for it make the game somehow dull.