I was typing while john was responding,,,so I'll just leave it
Well Hamm,,,I can relate....
There are a ton of mods that you
can do to this circuit.....it's a great platform for experimentation
You have come to the right place.....
You will get a lot of different responses when it comes to this question......some guys say "whatta ya need more gain for?"

Some will say "all
I would do is this".....and others won't get involved because you're the 1 millionth guy who has thought that he can improve upon this amp,,,,just after building your first amp

(me included

)
And a couple others know all the best stuff and won't give it up for free

You can get more gain out of the pre-amp as-is, without adding another tube (another stage).....adding gain adds noise, so you'll have to be careful with your lead dress and component placement
You probably don't need a switch to try different bypass caps......we use tools called decade boxes that clip in place of your caps and allow you to try different values....for now we can just give you different values to try and clip in one at a time....once you get above a certain value you won't hear much difference between 2.2uf, 4.7uf, 10uf,,, and you won't really need to install a permanent switch,,,unless you want to have a bypass cap "boost" switch that will just add or subtract the cap of your choice
BE CAREFUL!!!!!
guys get shocked and we don't want you to experience it,,,,so some guys will shy away just because they don't want to give you instructions on how to kill yourself..........being an electrician,,,I'm going to assume that you understand electrical safety
I'm listening to Nick Perry, but I have a feeling I know what you're looking for.....I recently modded a 4212 for a guy who is extremely happy with it now.....the trick is to not go too far