DIY 5th Gen 18,000 BTU Mini Split

This is a single split. This is where regulation bites. Under EPA Section 608 the refrigerant circuit may only be opened by a certified technician, and the EPA is explicit that this includes connecting or disconnecting the pre-charged line sets sold with "DIY" mini-split kits. No federal law stops you doing the mounting, bracketing and line routing yourself, but handling the refrigerant without certification voids the warranty and is a violation. Most US jurisdictions also require permits for a new outdoor unit: typically mechanical or building for the wall penetration and mount, plus electrical for the dedicated 240 V circuit and disconnect. An HOA nearly always has to approve exterior equipment in advance, and local noise ordinances can cap how loud the condenser may be at the property line. This varies by city and county, so confirm with your building department before buying hardware.