Looking for additional diagnosis suggestions, etc.
1989 245 DL MT5spd non-turbo 275K+miles

For many years ran with loosened coolant expansion tank cap due to one-time radiator leak fixed with a stop leak additive. Last year during annual state safety inspection, unbeknownst to me at the time, the technician helped out by topping-off the coolant in the expansion tank and fully tightened the cap. (No good deed goes unpunished!) Shortly after that I noticed “white” smoke (steam) coming from the exhaust after warm up, rust on garage floor, and rusting of the brand new exhaust system. Not sure if smoke/steam is due to coolant leaking only once engine warms or if only seeing smoke/steam as warming exhaust system releases trapped water.

1. No coolant in oil.
2. Coolant level has probably gone down but is happening so slowly as to make me question what I'm seeing. No detectable engine oil or brake fluid loss.
3. Spark plugs look normal for miles.
4. Boroscope (limited capability cheapie) doesn't reveal any obvious cylinder problems.
5. Compression test (cold) doesn't reveal anything obvious: #1 195 psig, #2 188 psig, #3 197 psig, & #4 199 psig. (Although cylinder 2 is lowest, it is well within what most consider acceptable range.)

Anybody experience this? Any thoughts for performing a better diagnosis? What else could it be other than head gasket? Leakdown testers aren't cheap – would it help diagnosis?
Thanks for any thoughts / suggestions