You have a Renix XJ (87-90) By design, the engine is not going to fire until the computer (ECU) gets a signal from the crankshaft position sensor and measures that signal to indicate that 300 rpms while cranking has been met/exceeded.
Somewhat extended crank times should be considered a “characteristic” of your engine, rather than a problem.
You can upgrade the battery and battery cables--at least 4 ga., 2 ga. better--and buy a high-torque starter.
You are correct in that good grounds are critical, especially for a RENIX!
Other than good basic tune-ups on a regular basis which should always be reviewed, (plugs, wires, cap, rotor, fuel filter) the above suggestions are things that will help optimize start times on a Renix engine.
99 Cherokee, 4.0 AW4, NP242
Past Jeeps: 49 Willys, 81 Scrambler, 88 Comanche
Without "data", all you have is an opinion!