Hi!
Rescue dating is best because they have the experience and often several boars for your boy to meet to find a personality match. There are a couple of different dating services offered, depending on the rescue. Rescue people will spot very quickly whether a match is promising or not.
Rescues (Adoption and Dating), Shops, Breeders or Online? - What to consider when getting guinea pigs
'Slow bonding' is basically a series of aborted bonding meetings they never get to process far, which is highly frustrating for the piggies and rather counterproductive from their point of view because boars need to start right back in square one with the bonding process every time they meet, so if you shut them down at the first sign of dominance behaviour, they can never really work through their own instinctive bonding manual - THEY know what they are doing, but not you.
In the end, it comes down to whether they are character compatible or not, just the same as in a quick one-off bonding process. That is why the success rate with slow bonding is not any higher because you cannot change the personalities and the outlook of a guinea pigs. They either vibe or they don't. There is sadly no magic wand trick that make all piggies get on, the same as with any arranged human marriage.
If you want to bring home a piggy and bond them yourself, then you ALWAYS have to have a plan B at the ready in case the bonding doesn't work out because the personality match is not there.
When rescue dating, your boy has full say about who they get on with and will only come home with a new mate if there is a match; so you do not end up with yet another single boar. You will also have the rescue at your back if there are problems with the bond. If you have access to a dating good welfare standard rescue (especially one that offers residential dating), then please go for it because all the risks are on the side of the rescue and not yours.
Bonding and Interaction: Illustrated social behaviours and bonding dynamics
A Comprehensive Guide to Guinea Pig Boars (see chapter companionship)
All the best!