I apologize in advance if this muddies the waters, but there is another possibility for how ship registries are assigned for Starfleet.
Let's say Starfleet assembles technologies and design strategies once every generation or so, to base multiple classes of starships on a "platform" of sorts; the individual ship classes may look significantly different from one another, but they may share components, ship-building/refitting techniques, or bundles of hardware, etc., designed and fabricated for their generation.
Some maybe ships in the 2260's are different than ships built under the previous generation specs (2240's). The "new" (2260's) ships have a different appearance (at least subtle changes) than the 2240's ships or the 2270s/2280's generation that follows.
So, let's say that the TOS/2260's era is the "current" generation. Components and shipfitting strategies are still on the drawing boards for the TMP generation. Now, let's assume that NCC-1371 Republic was of a previous generation, which specific one we do not know, but let's assume it was at least two or three generations prior to TOS. Let's also assume that FJ's numbering schemes in the Tech Manual are apocryphal for sake of argument. Maybe each hundred number series is either its own generation or each 200-number block is. So, let's assume that Constitution was the first starship of its class, NCC-1600, as a prototype in the 2230's or earlier. A handful were eventually constructed, maybe mixed in with other 1600 hulls or ships of various classes. Let's say the design was finalized in the 2240's, and by that time all the hulls for various starships in the 1600's. 1610s. 1620's, 1630's, etc. were taken. That doesn't mean that 100 starships were built, only that each sub-block within were already assigned.
Next, let's assume that once the Constitution class spec was finalized and the Enterprise was built, Starfleet was already assigning the 1700 block. This could be useful in explaining why Reliant was NCC-1864, making it possible that TMP-era Connies could also be registered somewhere in the 1800's. This would also explain how Excelsior wound up being NCC-2000 in the 2280's/'90's.
Let's say Starfleet assembles technologies and design strategies once every generation or so, to base multiple classes of starships on a "platform" of sorts; the individual ship classes may look significantly different from one another, but they may share components, ship-building/refitting techniques, or bundles of hardware, etc., designed and fabricated for their generation.
Some maybe ships in the 2260's are different than ships built under the previous generation specs (2240's). The "new" (2260's) ships have a different appearance (at least subtle changes) than the 2240's ships or the 2270s/2280's generation that follows.
So, let's say that the TOS/2260's era is the "current" generation. Components and shipfitting strategies are still on the drawing boards for the TMP generation. Now, let's assume that NCC-1371 Republic was of a previous generation, which specific one we do not know, but let's assume it was at least two or three generations prior to TOS. Let's also assume that FJ's numbering schemes in the Tech Manual are apocryphal for sake of argument. Maybe each hundred number series is either its own generation or each 200-number block is. So, let's assume that Constitution was the first starship of its class, NCC-1600, as a prototype in the 2230's or earlier. A handful were eventually constructed, maybe mixed in with other 1600 hulls or ships of various classes. Let's say the design was finalized in the 2240's, and by that time all the hulls for various starships in the 1600's. 1610s. 1620's, 1630's, etc. were taken. That doesn't mean that 100 starships were built, only that each sub-block within were already assigned.
Next, let's assume that once the Constitution class spec was finalized and the Enterprise was built, Starfleet was already assigning the 1700 block. This could be useful in explaining why Reliant was NCC-1864, making it possible that TMP-era Connies could also be registered somewhere in the 1800's. This would also explain how Excelsior wound up being NCC-2000 in the 2280's/'90's.