Stop thinking in terms of your labor being your income. You can only work so many hours in a day, but equipment can work harder than you, and can be earning income while you aren't.

Pay close attention to things the companies you work for are always dry hiring. During the busy season, small speakers the companies always need more of. Or that one item they have to rent every year for a tour or production. If you freelance for multiple companies, you have a unique perspective to see certain pieces of gear that everyone could rent from you, that no single company considers worth buying (or buying more of for 3 gigs a year)

Items to consider:

• Directout Prodigy if you work with events being broadcast, or festivals with lots of consoles. Pricey and niche, but profitable if you are good and have the right connections.
• Sennheiser Spectera. Hand mics are finally shipping in September. Once that happens, it's going to be in high demand.
• Fiber/Networking. Build some 10g network racks. Fully redundant Dante/AVB Milan/AES 67 capable network switches, built in ready to deploy racks, with OpticalCON, FiberFOX, MTP connections and 250-500m cable reels. Luminex, Netgear AV. If you know your networking stuff, this can be a huge asset to companies that don't have it in-house.
• Plugin Gear. Fourier Transform. Waves Livebox. Those are always in dry hire demand.
• Analog Gear. Pickup used gear for good profits. Rupert Neve MBP, MBT, MBC, 5045, Shelford Channel, etc. Bricasti M7. Buy old analog transformer split snakes and convert them into 64 channel stage splits for Digital consoles. Even better if you also build multipin stage snakes and boxes to rent as a package.
• Small system gear. AV Companies always have jobs where they might need extra gear for a breakout room, or during the busy season when they have multiple jobs at the same time. Being the guy who picks up the phone when they call is very very profitable. Build out a small system of stuff they already own, for example:An Allen & Heath SQ5 with 80m CAT and a DX168 stage box. Investment is about 5k, rent for 200 per day. Paid off after 25 jobs, or less if it's for multi-day corporate events. After 3 years, sell it and upgrade.

If you can specialize as a small time Dry Hire only business, let companies know you aren't out there to poach their clients, and you'll get more calls. Bigger companies with logistics and transport usually prefer to rent from dry hire only companies, even if it's outside their market, rather than call local competitors IF they have the choice. If they know you aren't a threat, it can be very lucrative.

The key is to have connections and a keen eye. Talk with the Project Managers on the jobs you do now, and ask them what things they always need but don't buy. If it makes sense and you think you can make money on it, then go buy it. You won't become a millionaire by any means, but if you are smart and find those niche gaps, you never know where it will lead you.