Yeah, that will be a lot more glow than you need under any circumstances. If it were me I would just put in a momentary switch and relay alongside the stock system. Running the plugs only when cranking will help but might not be enough glow to make it start the way you want it to. Running the plugs for a few secs before hitting the starter will make it light off immediately.
TDIs light up every light on the dash for a split second when you first turn the key on, so the GP light illuminates briefly regardless of whether the engine is hot or cold. However, the actual plugs will only get power when the engine is cold, or if they do get power when warm, it will only be for a split second. It doesn't need em in order to start.
Sounds like your engine is fairly normal. My '85 740 and '83 760 with D24Ts in reasonably good shape also will both crank a little longer when warm than when cold, especially if they have been sitting for a half hour or so. I wouldn't worry about it... or if you find it bothers you, just rig something up to light the glowplugs briefly.
Let us know what you find on the compression test.
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86 745 D24T/ZF 345k lifted 2.5"
83 764 D24T/M46 155k
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