You want to be a Paladin.
Sarenrae is served not just by clerics but also by paladins. It's a divine class that is far easier to fit with doing damage, and your rogue levels, and is thematically very similar to cleric. Most laymen would be unable to tell if a divine fire wielding priest of Sarenrae was a Cleric or a Paladin.
You specifically want 2 levels of Paladin, 3 levels of Sorcerer (Divine Soul, for preference), and then 2 levels of Fighter. 2 levels of paladin adds 2d8 damage from smite, and some additional damage from Bless (assuming you buff yourself before the fight starts/before you assassinate). 3 levels of sorcerer gives extra spell slots to smite, and Metamagic. Quicken Spell is expensive in sorcery points/spell slots, but allows you to Booming Blade as a bonus action, granting you an additional 2d8 + 2-3d8 + weapon + stat damage, from the extra attack. You can't do this a lot of times, but it's very good on a surprise round where you're Assassinating, as any extra dice based damage is doubled on a Crit, so Smites, Booming Blade d8's, it's all doubled on those rounds so spending points to get extra attacks and things is very worthwhile in that situation. Fighter 2 gives a fighting style (Superior Technique for another superiority dice, probs), which can add 1d6 extra damage, but more importantly it gives Action Surge - which once per short rest lets you Booming Blade yet again, adding another 2d8 + 2-3d8 + weapon + stat damage.
With your remaining levels you do w/e, you probably go fighter 3 for battlemaster and more superiority dice (meaning you miss much less, and get +d8 to any attack that hits + some rider effect), you probably go paladin 3 for vow stuff (pick whatever makes sense thematically), you then might as well go paladin 4 or fighter 4 or rogue 12 for ASIs.
Going straight to paladin 5 is also an option, and then getting fighter 2. That avoids using booming blade, and just attacks 5 times (2 from extra attack, then action surge for 2 more, then Quick Toss for a bonus action attack), and smites on all the attacks, for a buttload of damage (specifically, assuming you're using a Greatsword and everything hits, 15d6+12d8+1d4+15 or 30d6+24d8+2d4+15 when Assassinating :D), albeit probably only once per day (you don't have enough spell slots to do it more than once).
There's a strong argument for Eldritch Blast, too.
Although it's a bit weird and messy, Eldritch Blast starts getting pretty hectic at these kinds of levels. A Divine Soul Sorcerer 5 would have access to Quicken Spell as well as the third level cleric spell Spirit Shroud. If your magic initiate feat was switched around a bit to gain Eldritch Blast instead of Poison Spray, Eldritch Adept feat was taken (instead of Martial Technique? :D) for Agonizing Blast, an ASI put into cha, spirit shroud cast before combat, and you had 2 fighter levels on top? You could Eldritch Blast as a Bonus Action with Quicken Spell, and twice with Action Surge, for 12 attacks that deal 1d10+1d8+4 each, for a very competitive 12d10+12d8+6d6+48 damage (24d10+24d8+12d6+48 with Assassinate :^D). That would take you to level 18 - 2 further levels in Sorcerer would let you keep that Shroud up for 4 fights per day, and/or burn more spell slots on Quickening to keep the murder flowing. Tossing a level into Rogue for an ASI also works, to cap out cha and get that additional +12 damage (as well as +to hit).
If you're stuck being a Cleric
The sadly most optimized path is to simply take 1 level of Cleric (probably peace cleric) and just, not really use it for anything. After that take Fighter 2, Paladin 2, or Sorcerer, etc, like the above options but 1 level worse.
Rogue Rogue Rogue Rogue?
Staying rogue is a bad choice. It doesn't really give you anything, and the extra sneak attack or 17th level Assassin ability are all very.. meh.. compared to things like Smite or Action Surge. Even taking the 12th level to get the ASI doesn't help with much of anything. 11 levels in Assassin is, from an optimization perspective, probably at least 7 too many. It just doesn't give you many good abilities compared to dipping other classes.
