Trace all wiring from the battery, both + and - back to their source. Then check that those connections are clean and tight. You're talking about an AGM sealed battery? Some of those have high internal resistance straight from the factory and would have trouble starting an engine. You can get a serviceable electronic battery tester on AMA for less than $30. This is NOT just a voltmeter. Those are fairly useless when troubleshooting high-current problems like starting. The dedicated tester measures voltage, starting current capability, and internal resistance. That way, you'll know whether the problem is actually the battery and not a problem with the compression release. I'd be suspicious that the shop, from the way they were just throwing batteries at the problem without solving it, actually checked the valve clearances. They have to be set accurately for compression release to work. This is something you can do for yourself with just a few tools. Try it. There are many videos on YT that show how to do it. I recommend James Condon's videos. He's very thorough and shows in detail what he's doing.
