Creating a plug-in for existing trade software should be trivial. That said, I’ve been a software developer and systems architect for 40 years, so maybe I have a different perspective 
I’m writing mine in SierraChart as a Study because I really like their API. But I think Optimus Flow has a .NET API that could be used? I haven’t looked at it, but it might be something to consider.
I have already mapped out how each of the above functions would be implemented. For every order that is filled, a marker is placed on the chart. I select the marker and press a hotkey and that allocates the order to a trade group.
To identify a candle entry setup, I would place a marker over the signal candles and press a hotkey. The setups would be pre-programmed. For an entry based off a fib/horizontal level/ trend line, I would select the appropriate line and press a hotkey. For an order flow trade, I would place a marker on the footprint chart and press a hotkey. Each of the selected drawings would be grouped with the trade and recorded for later review.
I am trying to design the UI so I don’t need to leave the trading platform to journal anything; it is all done from the chart I’m trading from. It doesn’t interrupt flow, and more importantly it requires me to think about documenting exactly what I saw to enter the trade on the chart. I can add notes, and if I want a hard copy record, can create a .pdf of the screen as I see it with the relevant markings. I don’t think this would be possible with a separate journaling system.
The advantage of integrating with the trading platform is the user interface is already there and taken care of - which is often the most expensive part of the project. Another advantage is no need to interface with the data providers directly as the trading platform already does that - and then you can also use it for Crypto, Forex, InteractiveBrokers or whatever else the trading platform supports.
I am comfortable using SQL or Excel for analysis, so I don’t plan on writing a fancy graphical front end for analysis. If I decided I did want the stats to look fancy, I would probably use Edgewonk, but via a manipulated export that accommodated my trade groups and completed MFE figures.