"Currently, I plan to mount the antenna on the roof. Due to signal loss, I won't connect the coaxial cable to my second-floor apartment; instead, I will carry the radio up to the roof." — This sentence is unclear to me. What is the connection to lightning protection? In shortwave bands, it's not necessary to worry about coaxial cable losses. Even a thin 50-7 coaxial cable can experience a loss of only around 7 dB over a length of 100 meters. A 20-meter coaxial cable from the second floor to the roof is sufficient. This level of loss is barely noticeable, and using a cheaper 50-7 or even a 1/2 wave antenna will result in lower losses.
If the antenna is not within the protection range of nearby buildings or lightning rods, there's really no good solution. It essentially comes down to luck. You could install a coaxial lightning arrester, but that only protects against induced lightning; it won't prevent direct strikes. Alternatively, you could mount a grounding rod at the base of the antenna and manually ground during a thunderstorm, or simply disconnect the feed line. Ultimately, all these options are just temporary fixes.
Actually, based on my experience, using vertical antennas like GP within a city is not a good choice. The high ground noise and limited reception make it difficult to connect. Switching to a DP or inverted V antenna, even a shorter one, provides more connection opportunities than a vertical antenna.
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