Like you, I have found it most enjoyable to fly in DCS when I learn an aircraft properly, in-depth.
For me, this has meant limiting my focus to specific airframes: first Mi-8, then Mirage 2000C. Those have become so engrained in my memory now that I might be ready to learn a thirs module in depth. Probably the F-14 or upcoming Mirage F1.
EDIT: This is not to say that I don’t own a whole load of other planes, from the I-16, Bf109 and MiG-15 to the F-14 and FC3, but in those I just do some occasional circtuis and dogfights. No deep systems study.
Both Mi-8 and Mirage 2000C have excellent ‘teaching’ campaigns included, as well as good DLC for when you’ve finished the included campaign.
For the Mirage, I am not sure how well the campaign has held up with the various changes to the module, but the Mirage 2000C’s included Caucasus campaign did a great job of slowly teaching you the module, especially when combined with the training missions.
I think you can read the change log for the engine, radar and ECM systems and then just get going on the campaign.
When you’re finished, you will have gradually built up an understanding of the Mirage’s systems and are ready to do the Red Flag campaign, then finally fly the PG mini-campaign. All Baltic Dragon content, great stuff.
For the Mi-8, the included campaign is just VFR nav and basic weapons employment, but the Oilfield DLC really focuses on some of the different navigation instruments, from Doppler to various radios and how they can be used. Also a lot of sling loading. I love it! Haven’t tried the other DLC campaigns for Mi-8 yet.
From what I have read so far, Maple Flag campaigns aren’t great.
The highest quality campaigns for DCS seem to be made by Baltic Dragon and Reflected, so you can’t go wrong with those.