All of this took place without neglecting economy - I still had enough money to fully improve my economy each turn.
Try getting alliance with Hungarians and with Germans. This will protect your flanks.
No - just keep staying neutral with Hungarians (alliance is not necessary). To do this - do not conquer Iasi (in the video above I did - because this is one of provinces which can be captured very early, in turn 4 - but in my real, recent Poland's campaign I didn't conquer Iasi and left it for Hungarians - by doing so I still have peace with Hungary despite year 1300 and turn 111).
It is not possible to remain in alliance with Germans for longer time I think - so don't even try (in the video above I signed alliance & marriage with Germany in turn 4 - but I don't think it will last long, maybe if I decide to continue this campaign it will turn out).
In my current Poland's campaign (not the one from this video - which is just a guide showing how to conquer quickly during initial turns of the campaign) there is turn 111 (year 1300). I have conquered everything to the east and north of this line (included):
Including all these settlements. Russia is destroyed of course.
I also conquered:
Aleppo, Edessa, Baghdad and Mosul (in the Holy Lands),
Marrakesh, Timbuktu and Arguin (in western Africa),
London and Nottingham (in England)
And I only fought against Russia (destroying it), Denmark (after conquering their capital they asked for peace - I agreed) and HRE (war continues on).
I also fought against Egypt (crusade vs Antioch), Mongols (two battles for Aleppo), Moors (my "expedition" for gold and ivory) and England.
England (excommunicated) carried out a sea blockade of my port, starting war - so I asked the Pope to organize a crusade on London, he agreed.
Against HRE I keep 3 nearly stack (14 - 17 units) armies (in Hamburg, Magdeburg and Prague) - this halts their every attempt of conquest. When I see they coming closer to either of these settlements, I just send there reinforcements (to make this army full stack).
BTW - if they attack, they usually attack Prague (because this is a city - while the other two are castles).
I have alliance with Sicily and Venice (they asked me for alliance - I didn't ask anyone) and good relations with Hungary and Denmark (their faction heirs asked me to kill their kings - and by completing these missions I improved my relations). I also have superb relations with the Pope.
France is also quite friendly - I have lucrative trade with them.
In order not to provoke Hungary, I decided not to conquer Iasi (Moldavia) - which is usually conquered by Poland very early in the campaign. It worked well - I still have peace with Hungary (while normally Hungary attacks Poland sometimes even earlier than HRE and Russia).
Besides - Hungary conquered Iasi only after few dozen turns - it was more interested in expansion vs Byzantines.
Soon entire England is going to be conquered...
I will spare Scotland... maybe.
Keep Halycz as castle and Krakow as a city.
You should convert any settlement to city. Keep as castles only those settlements which are near your borders.
Cities produce much more income and also you can keep a garrison inside them for free - no upkeep cost (militia units).
That's why I usually keep as castles only settlements near my borders - and not even all of them.