best build for M! Robin?

Of course the easy way is TA and dual spurs.
You could give Robin a lancebreaker as B to take down beefy units that Chrom can't defeat.

Going for support/utility is probably the way to go, not making Robin scary, but doing that to his teammates

Well I think it also depend on what you want to do with him and what teamates he is gonna have appart from Chrom.
Don't forget that in order to make full use of his new refine you should use a tactic emblem, seeing that you have Chrom that is infantry you need the other two units to be not infantry.

Now here is what you could do :

IVs : +atk / +spd ( if you buff him with allies that might be good to avoid doubles ) and -hp or -res
Weapon : Tactical Gale + eff
Support : Reposition / Swap / Drawback
Special : Ignis / bonfire / moonbow
A slot : TA / Close counter / Distant def
B slot : QR / Guard / Bowbreaker / Swordbreaker / LanceBreaker / Chill of your choice
C slot : Drive of your choice / Close or Distant guard / Smoke of your choice
S slot : Distant def / Drive of your choice / QR / Smoke of your choice

For A slot I do think TA is one of the cheapest and best options as he doesn't have great stats but TA allows him to deal better with reds and colorless which is good. Close counter is good only if you plan on using other units to buff him ( i would advise to use mostly drives to stack on the whole team in addition to his own buffs ). Distant def can be good as you can tank some blue mages aswell but overall more expensive and not essentially better than TA.

For the B slot, QR is obviously very good as it allows him to double on most of his matchups and kill with a special ( or get it ready for next kill depend what special you choose ). Guard paired with QR seal is an other option. And chill skills can be amazing if you want robin to fill a full support role.

For the C slot, Drives are amazing in order to turn him into a better buffer, Close / Distant guard can be good aswell. A smoke skill can work if you plan on using him to bait ennemies and let the rest of your team deal with them ( especially good against all those B!Veronica as Robin can tank them and debuff them ).

For your seal, you can either go with a second drive for more support. If you chose smoke as your C, you can get a second smoke here to debuff further.
Distant def is good for more ranged tanking. If you chose close counter, you might wanna get close def here.
Finally QR is nice if you picked a guard skill or a chill skill as B slot.

Hope that helps :)

One time I saw a +10 Robin in arena with Blessed Bouquet+ and Close Counter

If you want him to be a frontline unit, his refine and weapon will need to be swapped out. His speed is really lacklustre in today's speed tiers, so I'd use that as his bane, unless building specifically for it. His best stat is his defense in this meta, so I'd capitalize on that, personally.

The two sets I'd recommend are either Blarowl+/Blessed Bouquet+ (def refine for both) with Bonfire, CC, QR and CD3 seal. He can hit pretty great defensive statlines, but he needs CC for those to be most effective.

The other set I'd reccommend would be Blarowl+ (speed refine), with Bonfire or Blue Flame, Atk/Spd Bond (or another bond), maybe Chill Speed, and then the Atk/Def Bond. He'd hit decent attack, would still be tanky, and would be a much better all-rounder with his stats. However, he'll be up against mages a bit more, which means he'd struggle due to his weak resistance.

If you really want to use his native tome, he'll really struggle against anything not red or colorless, and he'll only be able to deal with them if he has TA, because for him to be most effective you'll want to load him up with skill to support others, and he won't get much himself. Full on support with Tactical Bolt (+Eff), Triangle Adept 3, either a Chill or a Feint, either dual drives or a dual spur and a drive, and a good Rally to top him off.