Multi-region equestrian (US + UK + EU), what changes per region?

Equestrian regional differences run deeper than typical e-commerce:

• US, Imperial (inches, lbs), USD, dominant disciplines: hunter, jumper, dressage, Western (barrel, reining, cutting), endurance. Major brands: Dover, SmartPak, State Line. Tax: state-level sales tax; resale-cert exemption for stables.
• UK, Imperial-ish (hands, lbs but cm + kg often), GBP, dominant: dressage, eventing, hunter, polo, hunting. Major brands: Derby House, Horseware Ireland, Joules. Tax: VAT-included pricing. Compliance: SE-Eq (Society of Master Saddlers) qualified-fitter recommendation for saddles.
• EU, Metric (cm, kg), EUR, dominant: dressage (DE / NL / SE), show-jumping (FR / IE / BE), endurance (FR / ES / IT). Major brands: Krämer, Loesdau, Equiline, Pikeur. Tax: VAT-included by country (DE 19%, FR 20%, NL 21%, IT 22%). Compliance: EN1384 helmets, VG1 helmets, CE-marked tack.

Magento Multi-Source Inventory (MSI) handles split warehouses (US + UK + EU). Store-view per region for currency + tax + language + measurement units. Same SKU pool, different price visibility, different checkout (PayPal in US, Klarna + iDEAL in EU, Clearpay in UK). hreflang annotations for organic SEO; canonical per region. Two-warehouse split (US + UK) covers ~85% of cross-Atlantic equestrian; add EU warehouse at $5M+ EU GMV.