Questions ragarding Deathmatch and improving overall

A little bit of context:

I played the Standard Random Map game mode for over a year against different AI opponents (most of the time, because my internet was bad). I became noticably better, but I'm also hitting a "soft cap" in the learning process and now I don't get better as fast anymore.

By now I win with ease against every AI that is weaker than the Resonance Bot on hard difficulty (especially after the nerf to the HD AI on normal), both in teams and 1v1. "Hardest" is still out of my league. That I just can't seem to consistently beat the Resonance Bot on hard is a bit frustrating and I thought a chance of pace might be good for a while, so Deathmatch came to mind.

Question time:

• What would be the most efficient way of improving overall from this point onwards (both in learning Deathmatch and on default Random Map mode)?

• How do the different Bots perform on Deathmatch? (and which difficulty setting should I start with there?)

• Which starting Age is the best for Deathmatch? (the wiki says Dark Age, youtubers say Post-Imperial)

• Why are Deathmatch games only played on fast speed? (I prefer normal speed on Random Map, because it gives me more time to think and makes the bot's omnipotent micro less op)

• Are there any custom maps that I should get familiar with before playing Deathmatch online?

• How well are the civilizations balanced on Deathmatch? (and which Starting Age makes for the most balanced games?)