Hi there, very interesting and very nice bike you got there 
A 1x chainring can be an upgrade, but isn’t necessarily one, when it forces you to cross chain or pedal at an unnatural cadence. When done/applied correctly, it can save a few watts here and there.
Speedplay Pedals - in case you can do with a narrow q-factor, buy the old „Speedplay“ ones, not the new Wahoo ones. The new ones come with a long spindle and the old ones with a narrower one. The narrower one will be a tad more aero and also makes me more comfy. I ride normal Speedplay short spindle pedals all year round and the aero ones just for racing. The dual entry is just too comy.
Overshoes - definitely a solid choice. If you are not planning on participating in UCI-sanctioned events, CTT ones are great. NoPinz is a great choice also, the long ones go all the way up to my patella.
A Trainer - a smart trainer or a coach? Either way, loads of improvements to be had.
Front Wheel - I literally like anything better than the CLX64. The Troika MAX (TMD is a prototype version of the MAX) is a very solid choice. I can also recommend the 2020/21 DTSwiss/Swissside 80mm wheel. Super stable in windy conditions.
Not planning on anything in Southern Germany really - my A+ event for next season are the „Northern German Championships“, so the opposite end of the country. When I go to the Alps, I bring the climbing bike, not the TT bike 