Q1: that previous thread includes a link to
Shop Square D Load Center Hardwired Surge Protection Device at Lowes.com but would that not work? It's 175 VAC max. Is something else recommended?
Your garage would have numerous and distinct grounds as described by FlasherZ. Even if interconnected, those grounds would be unique. That is required for human safety and described by code.
Your Q1 is about something completely different - transistor (or Tesla) safety. If four AC wires enter a garage, then all four wires must connect to earth where entering (ie at the sub-panel). Obviously you cannot connect all wires directly to earth ground. So something must connect a wire, as short as possible (ie 'less than ten feet'), to earth ground (ie those 6' separated ground rods).
175 volts is an irrelevant number for that Square D (and other recommended) 'whole house' protectors. (In reality it is a 330 volt device as listed on the box.) For surges, an important number is current. How many amps can the protector connect to earth and still remains functional? 50,000 amps is a typical minimum number. All protectors are only connecting devices to earth ground. You cannot connect those four incoming wires directly to earth. So a protector must connect as much as 50,000 amps to those earth ground rods.
Can a 200 amp service carry a 50,000 amps surge? Of course. And even higher currents. 200 amps describes AC power. 50,000 amps describes a completely different current - surge current. Your service must be rated for 200 amps AC current. Your 'whole house' protector must connect up to 50,000 amps from those wires to earth ground.
If less than 50,000 amps is not connected to earth where it enters a detached building, then current will find earth destructively via items inside. In all surge protection, a surge is either harmlessly earthed outside the building (where it would enter the building). Or that surge finds earth destructively inside. So that a surge need not pass through a Tesla, the surge must be earthed where it would enter the garage. That is what protectors do. Connect a surge harmlessly to earth.
Every foot shorter to earth means increased protection.
That Square D is not a 'whole house' protector. Square D makes a 'whole house' protector that would connect three of four incoming wires to earth (the fourth is connected directly; therefore needs no protector). Other manufacturers were listed in that previous discussion.