Hold Mark is currently the only way* (for an alchemist) to get critical specialization with bombs (as of early November 2020). That doesn't preclude Paizo from publishing such an option in the future.
There are several ways for non-alchemists to get the critical specialization effect of alchemical bombs (see the Swashbuckler's Weapon Expertise, for example). However, alchemists eventually can expand their splash radius to 10 feet with Expanded Splash at level 10 or the Bomber's Greater Field Discovery at level 13, which would obsolete the critical specialization anyway.
For reference, I looked through every instance of a feat that Archive of Nethys found as having both "critical" and "specialization" in the text. I also examined the multiclass archetypes to see if any granted access to features of their base class that would grant a critical specialization to bombs in some way (e.g. via critical specialization with all martial weapons). It is obviously possible I missed something.
*@Delioth mentioned another possibility; the Owlbear Claw talisman. This is an incredibly expensive option, but should work. You would have to attach these to the bomb before you threw them, so they could be "expended" on a hit or even a miss. They might be retrievable after the fight, as the Gamemaster's discretion. You pick up the Talisman Dabbler archetype to get some of these free every day, but it's still going to be tricky to make it work. You could perhaps ask your Gamemaster to allow you to fix these to your throwing appendage so that you can only expend them on a critical, but that is outside the realm of RAW. I don't think it would be unbalanced by any means though.