Hi, I’ll try to answer even though I’m really late here. I’m somewhat new in developing scenery, since the past year, mainly in previous gen sim (~P3D).
My building detail is high at where it needs detail, e.g. main terminal, apron, jetway, etc. It’s where sim players mainly wandering around. Other extras I made them low poly count because it’s mainly just there for immersion, just a few hardcore players that went to the toilet inside MSFS 
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For textures, I use higher pixel count at those detailed building, and smaller texture size for extras to conserve GPU memory. Also consider draw count (number of textures that need to be called when sim is rendering the airport). Obviously smaller means higher FPS, just balance it to quality.
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I used sketchup for the past ~15 years. I’ve never used bevel edges, but if that means better quality why not try it? Adding a plane roughly only increase 2 polygon count. Not that much considering a study-level aircraft cockpit will have hunderds of thousands even millions of polygon. I don’t think it’ll give high impact on performance.
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Try to keep poly count lower. Add it if you want, but don’t give too much detail. Good outline will do.
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Just play with texture, you can use same UV map but different markings, colors, etc. Not that much of an impact for a small airport.
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I only design every object once, since it’s already a burden to work on one haha. I try to keep poly count reasonable, like doing not too detailed bevel, choosing which object to be recreated in 3D or just 2D, classifying the importance of object, should I design it well, or just simple extras. You’ll understand along time, and by trial and error, looking at your finished design inside sim and doing correction here and there. Trust me it’ll take a long time to create something good.
Last but not least, don’t stop learning and don’t be shy to ask. Sometimes people are good, sometimes people are lazy to help. But if you don’t ask you’ll never know