Simply using BR to limit participation feels very artificial when there are 25 1st rates available in the nation. Much better to implement changes that have follow on effects that limit the total number of 1st rates in game, which cold include...
1. Slow down crafting of rated ships, in terms of cost/resources and time to craft.
2. Only allow Admiralty to hand out BPS for rated ships, and limit the production run of those BPs.
3. Only allow rated ships to be based at specific ports, such as regional capitals.
4. Introduce supply requirements.
5. Reform the rank system so that nations. aren't so top heavy with Admirals.
6. When land in battles is added, include harbour depth to limit the utility of deep drafted rates.
7. Include variable wind conditions in battle to increase the utility of light ships that can respond better.