I don't think that the Betazoids have the economy to really make best possible use of swarm doctrine-though they probably don't have the maximum power to make use of everything
They do. The Betazoids can produce a Centaur-A every 2.67 years with their current income. Factor in that they only have 1 shipyard capable of producing them and currently have 155BR, the limiting factor for them, they can produce a Centaur semi-annually for the next 14 years. By that point their long term goal of "improve budget" should have kicked in and they could probably continue producing one every two years forever.
Even putting that aside however the names of doctrines are
terrible for determining their usefulness. Swarm actually benefits very little from outnumbering the enemy. Since people keep getting confused here is a full list of what Swarm and Combined Fleet provide:
Swarm Doctrine:
0 / 25 Sting Like a Bee (+5% Fleet Combat when outnumbering)
0 / 25 Multiple Attack Vectors (+4% Shield Burn-Through Chance)
0 / 25 Float Like a Butterfly (Escorts gain +10% Evasion)
0 / 25 Live Another Day (+2 to Defence when attempting to flee)
0 / 25 Quantity is Quality (+2 to Escort Research, -1 Qtr to Escort build times)
0 / 25 Street Patrol (+1 to Escort Response Rolls)
0 / 75 From Many... (+1 responding Ship able to add stats to event rolls)
0 / 75 ...One (+1 to Escort Response Rolls)
Combined Fleet:
0 / 25 Mix and Match (Sector fleets with at least 1 Escort, Cruiser, Explorer count requires -2 D)
0 / 25 Combined Strike (+5% Fleet Combat with at least 1 Escort, Cruiser, Explorer)
0 / 25 Broad Push (+1 to all Ship Design Research)
0 / 25 Diversity (+2 to all new class design projects)
0 / 25 Screening (Explorers in a fleet with more escorts than explorers gain +10% Evasion)
0 / 25 Ablative (Explorers in a fleet with more escorts than explorers gain +1 L)
0 / 100 (Cost of new class design projects reduced by 50%)
Swarm Doctrine is actually all about providing bonuses to Escorts with only one bonus related to fleet size. Combined Fleet meanwhile requires Explorers, which are
definitely outside the Betazoid's budget with it taking
eight years for them to afford a single Excelsior, for 4/7 bonuses and literally all other bonuses are related to ship design, which is something the Betazoids seem to be abandoning in favor of using Starfleet designs.
Frankly speaking
Lone Ranger is better then Combined Fleet for the Betazoids since at least they'd get some benefits out of it without needing an Explorer.