What matters at portfolio scale
Managing 5 domains and managing 500 are different problems. Past a certain size, the registrar decision stops being about the checkout price and starts being about API access, bulk renewal controls, and how much friction there is to move domains in or out — the same scale problem covered in building a portfolio that outgrows a spreadsheet.
Namecheap
Straightforward bulk management UI, reasonable renewal pricing, and a documented API — the default choice for investors who don't want to build custom tooling. Support quality is inconsistent at high volume.
Porkbun
Consistently among the cheapest at both registration and renewal, with free WHOIS privacy included. Bulk tools are thinner than Namecheap's; better suited to a portfolio you're not constantly restructuring.
Dynadot
Built with bulk investors in mind — folder-based organization, bulk push/transfer, and an API that's actually used by third-party portfolio tools. The interface is dated but the bulk operations are the most complete of the three.
What to avoid
Registrars with aggressive upsell flows during checkout or renewal (extra "protection" products bundled by default) cost real money at scale — a $2/domain upsell across 300 domains is $600/year for nothing. Also confirm transfer-lock and auth-code retrieval aren't gated behind a support ticket; you want to be able to move a domain out in minutes, not days.
The practical setup
Most serious portfolios end up split across two registrars: one for the bulk of holdings (cheapest renewal, decent API) and one kept separate for premium or high-value names, so a single account compromise or registrar-side issue doesn't put the whole portfolio at risk.