Regarding the pool - that will most likely be the most economic option for your town. It doesn’t hurt to ask about alternatives, or to inquire about their solvency. Someone familiar with regular P&C insurance, but unfamiliar with public entity pools, would turn green at the capital levels supporting the pools Many of the pools reinsure their exposure, which helps, but when I first looked at assuming pool risk…it was educational. (We didn’t normally try to compete against the pools, because of their cost advantages and their not needing to generate profit.)
SIR vs deductible - the two practical differences for liability from the insured’s perspective are cash flows and claims handling.
If you have a deductible, the insurer does all the work, bills you for deductibles, and perhaps requires a letter of credit or collateral to reduce their exposure to credit risk from you.
If you have an SIR, you handle the small claims (e.g. fender-benders, plow vs mailbox incidents), or contract with a TPA to handle those claims. You notify the insurer when claims reach a certain size, and the insurer becomes involved when there’s a risk of exceeding the retention.
IME $25-$50k retentions are where SIRs start to make more sense than deductibles for auto liability / general liability / professional liability. However for your sized town, you’re probably looking at a deductible of no more than 10k. An SIR might not even be an option from the pool.
(Disclaimer - I don’t have public entity experience with Mass, since that is a state where pools are dominant. I might have looked at assuming pool risk there once…)
Re auto liability: probably not a consideration for you, but in a couple of states, statutory immunity is so strong that governments don’t normally get auto BI/PD coverage. However, when they do that, they either cannot take their vehicles out of state (statutory immunity won’t apply across a state line), or they will buy auto liability coverage for the out of state risk. I don’t think that’s a factor in Mass…but I am not certain.
(Public entity P&C insurance practices vary considerably from state to state, and are heavily influenced by custom, sometimes in ways frustrating to the actuaries pricing the products.)
Re jail exposure - I mention it because jail exposure is one of the ugliest parts of the public entities P&C segment. (Coverage for schools is also gnarly. I hate school sexual abuse claims, and rebuilding schools after a total property loss is hell from the insurer’s side.) Hopefully things have improved since I rotated away from public entities, but…
One more point to educate yourself on - which liability coverages are on a claims-made basis, and which are on an occurrence basis. You probably don’t have the option to pick and choose, since tradition and carrier offerings will drive the decision. If you move claims-made coverage to a different carrier, or in the extremely unlikely event you switch a coverage between occurrence and claims-made, ask about potential coverage gaps. The broker will probably be on top of that, but better safe than sorry.