http://www.stardestroyer.net/Empire/.../Asteroid.html
I usually default to ESB asteroid destruction for current canon SW which Mike Wong (admittedly a biased source but from what I know having appropriate technical knowledge) conservatively estimated at 92 TJ or 22ish kilotons per shot. Notably an Imperial-II SD (the kind seen in ESB) has 60 of those smaller point-defense guns in current canon as well as 8 larger turrets.
So a Systems Alliance dreadnaught can fire at roughly 32 kilotons every 3 seconds in-lore and it takes about 3-4 dreadnaughts to overwhelm a Sovereign-class Reaper’s kinetic barriers IIRC according to the Codex. So roughly 128 kilotons every 3 seconds, 256 kilotons every 6 seconds.
The larger batteries (albeit on the older-model Imperial-I SDs) as seen in Thrawn’s orbital bombardment* in Rebels can roughly fire a 2-shot burst every 2 seconds or so (I counted roughly 11 shots in about 6 seconds from the port-side battery by slowing the clip to .25x speed.
https://m.youtube.com/watch?v=ARR0RPrr_rg
8 batteries x 22 kilotons averaged over over 11/6 = 322 kilotons in 6 seconds, comparable to about 5 Alliance dreadnaughts.
Notably, that is just the big guns, and that is assuming the big guns have THE EXACT SAME firepower as the small point-defense guns while also ignoring said 60 small turbolasers and the 60 ion cannons an Imperial-II is packing. And as you mentioned a Reaper’s shields may do less to stop turbolasers than pure kinetic weaponry.
By raw firepower alone, for my money a Sovereign-class Reaper wants none of even a Disney-canon ISD, to say nothing of the Executor-class SSD.
*Which it should be noted was not intended to be lethal as he had orders to capture the Rebel leadership, noted since the long-range firepower is clearly not kiloton range.