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Garage sub-panel

I want to upgrade the wiring in my detached garage. It was built in 1961 and has a 3-wire #6 cable (2 hots and a neutral, no ground wire) run from a 30 amp breaker in the main panel in the house to a small fuse box in the garage. From the fuse box there is a 220v outlet plus (2) 110v circuits with screw in fuses. The existing wiring is a mess, hacked by previous owner, chewed by rodents, no grounds so it all needs replacing.  I want to install a new sub panel, delete the 220v outlet and have (4) 110v circuits for lights and outlets. No large loads anticipated, just standard hand tools and such, separate circuit for a door opener, mostly LED lighting. It is close to 100 feet from the main service panel to the garage panel.

Current code requires a 4-wire system for new work but has an exception for existing systems. When I poke around on the interweb for 3-wire systems, I get a little confused on how to do this in terms of grounding. Once I have the sub-panel hooked up the circuit wiring is straightforward. Different people seem to have different ideas. Looks like I need a dedicated ground rod (or 2) for the garage and it would be connected to the same bus bar as the white neutral feed wire. The feed wire comes thru a non-metallic underground conduit and there are no other metallic paths bonding the 2 buildings. The feed line has a standard breaker, no GFCI. This is a rental and running a new 4-wire feed would be physically and fiscally painful.

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