Back to your slow starting problem, do you know what the compression ratio is in your engine? If you don’t know that, then maybe you could put a compression tester on it and let us know what the gauge reads? (One cylinder should do )
I’m wondering whether the starter is working really hard to turn it over because of high compression, or the starter is being starved for power due to inadequate battery/starter cables or poor cable connections.
I run #0000 welding cable ( often called 4/0 AWG ) for BOTH main positive and ground cables. Connectors on both ends of the cables are swaged-on for the best connection. Welding cable is very fine stranded, making it very flexible, but because it is fine stranded it can pass more current, the thing that provides power to your starter. The bad news is that for a 10’ length you’ll pay $75 - 100 bucks. But if it cures your hard start problems……..