JTR team without R2...

you might wanna reconsider and put r2 with JTR

but to answer the question a droid or resistance member should replace him- resistance for the team synergy, (all of them bring something nice to the table except like, pilot and rose) or droids to give bb8 more inital tm, meaning JTR takes her turn faster and the tm train starts a little sooner. Droids to avoid- 86, GG, Magnagaurd. You probably don't want b2 or k2 either. And might wanna leave chopper for pheonix

Id say Holdo or Poe or Nest. Holdo has a heal and works on D. For offense Id say Poe. Nest would work on either.

Well...
For TW, R2 must be joined at JTR squad.
Please think about this again.
For TW... There are lots of great rebel units like RHan Chewy Chaze..
But Resistance.. without R2, you cannot beat meta-squads very well.

For TB, JTR doesn’t need R2.
TB is easier than TW.
I clear 6/6phase mission w/ JTR ver easily.
G12 Jedi-Training Rey (L U1 U2)
G12 BB-8 (U1)
G11 Scavenger Rey
G11 Resistance Trooper
G9 Finn
And R2 is used w/ CLS.

L3-37 is an unorthodox but viable option. She's a great tank and will synergize pretty well with BB8, even without any Scoundrels present.

All things considered though, I'd probably pick Holdo, assuming you can stomach a 20-energy hard node farm for a character you won't use a ton.

Holdo was great for me in arena during the EP meta.

I’d have more flexibility for my Rebels if I had farmed Chaze.....but that ship has sailed! My R1 team are the 5 pilots (Rebel Fleet).

I never use R2 with JTR in TW or TB (or raids, for that matter) because I find he brings more to a Rebels squad than to JTR Resistance in those areas. My usual TW/TB Resistance team is JTR, BB8, Poe, Trooper, Scav Rey. I stick R2 with CLS teams most of the time.

I use R2 with JTR because I field two Resistance squads and there's only 10 Resistance toons. He might bring more to a Rebel squad but there's plenty of other rebel options.