Ok I have a little bit of time on my hands, not much, but enough to address some of this
wrap a string around each tire, KNOW the circumference then make adjustments IF needed. just because the numbers are the same, (or different) on the side of the tire, means they are the same (or different) upon real measurements. Ski
The overall geometry of the tires circumference when measured in a static setting is not going to be the same measurement when the bike is in under dynamic load and/or when the tire flattens out when riding on the edge of the tire which decreases those measurements. Just because you know the diameter of one tire to another it does not automatically mean those differences are the geometry adjustments required to maintain similar handling characteristics. The profile of the tire belting affects this just as much if not more than the actual tire size. In fact the handling of the bike when it is straight up and down is rarely the focal point of these kinds of geometry changes anyway as it is the size of the tire when it is leaned into the turn riding on the edges that cause the handling issues. We adjust the geometry to get the bike to follow through on corner exit or to not fight us on corner entry while trailbraking etc, but at the same time we are searching for that sweet spot that allows the trailbraking of the bike to alter the geometry in a manner that is conducive to the natural movement of the bike into and out of the turn. We do this by adding or removing ride height and making sure the damping rate is stiff enough to not allow excessive dive from the front end during those maneuvers, but rather facilitates the turn-in by offering proper front fork ride height under braking. Many riders want their bike to handle in neutral fashion where it does not try to fall into a corner or fight you requiring constant input into the handlebars to stay leaned into the turn. A neutral handling bike will basically allow you to let go of the handlebars in the middle of a constant radius turn and the bike will complete the turn on its own.
Ultimately it is impossible to truly tell someone how to fine tune their geometry as it is dependent on a myriad of variables not the least of which is what kind of riding they do, how much they hang off, how fast they are, what brand/size of tires and what type of feel and feedback they expect from their bike to give them the confidence they need. We can get them in the ballpark, but when it comes to the fine adjustments like the OP is asking about there is not really much any of us can add to his inquiry that he is not going to have to figure out by trial and error or in some cases someone might just get lucky in that their advice is close enough for him to be comfortable with.
I'd just add a click or two of compression and rebound to compensate for the angle and go from there. option 2, ad 5mm shim to the shock and call it a day to keep the numbers the same.
I'm assuming you already had your suspension dialed in though..
I can tell you that you do not alter the static geometry of the bike to compensate for more or less ride height by changing the damping settings. Nor do you just arbitrarily tell someone to add 5mm shims to their shock mount etc. If his bike was handling properly originally and he had a 6mm shock at the clevis which adds roughly 18-24mm of ride height at the axle then adding another 5mm shim would double his ride height at the axle.
There is a ton more to discuss on this topic, but I've burned up my free time so hopefully some others will chime in and fill in the blanks I have left or further stimulate the conversation and we can continue to discuss it later.