I have started a new game as USSR and have played until mid-1938! Some basic observations:
1. The purge is very static and little effect, some 6 generals/ministers removed and 1% dissent hit. I remember in vanilla there was a choice about whether to do the purge or not? Also in vanilla I remember the event being more extreme (more leaders/more dissent cost).
Absolutely right!
Since the stated goal of my Mod is total Historicity and Realism, I completely reworked the Purge event, removing alt-historicity and randomness.
- now the Purge takes place not with one event, but with three events (Tukhachevsky and his generals are eliminated in 1937, the main Purge happened when it finished with the elimination of Yezhov - at the end of 1938, and air-generals eliminated before the War in 1941)
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Only those generals who were really repressed are eliminated.
That is, I checked every Soviet general for his death and those generals who were killed in 1941-1942 in my Mod also die in 1941-1942 (if Germany manages to capture certain provinces in those areas where these generals were really killed)
In vanilla, a lot of generals who actually died on the battlefield in 1941-1942, for unknown reason were harshly inscribed in the "dead-list" of the purge event. That is, I gave them a chance to die in battles by their historical death in historical years, in historical provinces.
Therefore, be ready for the fact that as Germany captures the Soviet provinces, some Soviet generals will die.
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About a small dissent for Purge in my Mod.
I think the idea of dissent growing after you massively shoot dissidents is rather illogical.
My simple dictator's logic tells me - the fewer living dissidents are, the less dissent is!
Therefore, I do not understand the logic in the traditional for Hoi +10 dissent for Purge.
On the contrary! After the Purge, everybody is
totally intimidated and no one dare to open his mouth against the authorities, or even to work poorly and lasy.
In reality, the Purge did not reduce the USSR's GDP by -10%, this is absurd.
Therefore, I give a small dissent fo Purge - just for the fact that the bravest who survived are still quietly whispering their damnes... hiding under a blanket in darkness and when no one hears them.
But at the same time, the Purge in my Mod has serious negative effect on the quality of the Soviet army - it reduces the GDE and shifts the Army slider to a worse side - such effects really correspond to reality.
2. The Caspian Sea Flotilla is a waste. Even though there are ports in Baku and Astrakhan, the fleet refuses orders to dock there. In the end, I ended up disbanding them immediately as they are useless without being able to move, unless the intention is to have them act as a bridge for 1 division at a time to go from Azerbaijan to Kazakhstan?

That's player's right!
I made the Caspian Sea transportable and put 1 transport there for maximum historicity and beauty, not for it would be imba.
But in fact, I myself sometimes got into a large encirclement in the Caucasus and saw how other players got encircled in the Caucasus (in Multiplayer games), so having 1 evacuation transport to try to get several divisions out of the Caucasian encirclement is really not bad!
Unfortunately, in that MP game where I was surrounded in the Caucasus, I did not have this life-saving transport.
To be honest, at first I made the Caspian Sea with normal ports (not just only with beaches as it is now), therefore more transport-units could be unloaded there.... but during the test for Germany, I faced an insurmountable problem - as soon as the Axis reached the Caspian Sea, Italian convoys in the Mediterranean were immediately diverted to the Astrakhan port

, which completely paralyzed the supply of Africa. (before the Axis reaches the Caspian Sea everything worked fine)
I couldn't figure out how to fix this bug, and therefore in the later versions I was needed to remove ports from the Caspian Sea and leave only "beaches" through which it is nevertheless possible to land/unload divisions.
I assume that in case of encirclement/threat of encirclement in the Caucasus, this transport have a chanse to save several Soviet divisions before this transport will be destroyed by Axis aircraft.
3. The Spanish Civil War auto-win event needs some finetuning. It gives automatic victory to the Nationalists, but I used the (very good to have) choice to take military control of the Republicans. With some encirclements, I destroyed enough Nationalists to make us even sized, and a stalemate developed where I had 30 provinces/VP of Spain and they were bottled up in Burgos with 9 provinces, but they still won by event.

lol
Some of my MP players also tried to manage to win in Spain for the USSR, until the end of 1936, when the harsh multiplayer event for Franco's victory happend, but no one has ever managed it yet.

This is a kind of challenge.
You know, I totally agree with you!
Probably I should deactivate this multiplayer "auto-win" event in Spain, for the case of a Single game. I really hadn't thought of that. Another time I will deactivate this event for the Single game case, as I deactivated some other multiplayer events for SP.
The multiplayer unhistorical event for the "automatic victory of Franco in December 1936" was made so that in a multiplayer game the Axis player could take Spain immediately after this event and to develop Spain since 1937. (in our old multiplayer tradition - we don't waste time on the wars against AI until 1939, so as not to waste 10 players' time on boring waiting and pauses, we try to make the pre-war years pass as quickly as possible)
But for a Single playing, this MP event really needs to be deactivated, thanks for the comment!
4. You mentioned some difficulty options for the German war, in the game based on selecting normal difficulty. I tried this game with normal difficulty but didn't get any event popups at the beginning. Will they come when the war starts or did something go wrong?
Don't worry, a window with a choice of difficulty will appear at the time of the German attack on the USSR.
This is done on purpose so that the player could replay the game with different levels of difficulty, without necessarily having to start the game again since 1936.
It will be possible just to reload the pre-war save for the USSR and try another difficulty.
I also tried to make check that the player play historically and did not commit aggressive attacks on neighbors and non-historical annexations.
Therefore, if a window with a choice of difficulty suddenly does not appear on June 22, it means that the player was having some kind of non-historical aggressive wars.
5. There is nothing about the this war in the mod? Surprising, because otherwise there is very good detail for many countries.
Agree, it's worth thinking about to implement it.