Since you have differing relationships with the band’s members, consider approaching breaking this news differently as well.
RG you have a strong relationship and long history with. He knows
your musical tastes and it is quite likely that he will have seen
LV and D it seems you have a briefer and comparatively less involved
relationship and B is a bit of an unknown quantity.
Thinking about what is appropriate in each of those scenarios I would suggest something like:
Tell RG first and privately, preferably not at a practice session or gig, arrange to meet him and let him know you have something you want to talk to him about. You owe him that respect and a one on one conversation between you about it can be more in depth than with the rest of your bandmates.
Make sure that you preface the news with an expression of appreciation for your common history perhaps something along the lines of :
RG, we go way back and I really appreciate everything I’ve learned
from you and the stuff we’ve shared. I really hope that what I need
to say next isn’t going to change any of that.
This gets across both the value you place on him and the fact that you aren’t making changes on a whim, but because they are important to you.
If it is appropriate, you could then present your decision as being part of your maturing as a musician, which you couldn’t have done without his support and guidance.
I won’t try and script that because you know what is appropriate to your relationship better than anyone, but make it heartfelt, make it honest and also make it obvious that you assume he will receive the news with goodwill for your future ventures
I know you’ll wish us well, man
People like to be thought well of and often try to fall in line with good assumptions about them, even if they don’t entirely feel that way in the moment.
Make it clear that you are more excited for new opportunities than focused on rejecting the old experiences.
If he takes it well, ask for his advice and possibly help in telling the rest of your bandmates. It might be good to give him a few days to think about forward strategies for your old band before you tell the rest of them, if he takes that kind of role in the band structure.
Consider also discussing with him at this stage whether it is best to fully leave the band as soon as you have told the rest or to offer to stay on for a while, if there are any upcoming gigs that would otherwise have to be cancelled. Just be careful not to get drawn into a rolling leave-date which never actually happens.
A band practice is probably the best time to tell the rest of the band, that way they are together and can discuss what it means for them once you have broken the news.
How you actually word it will depend how you got on with RG, but be clear and concise. You need to tell them what you are doing, briefly why and that you wish them well:
- you want to take your music in a different direction,
- you’ll miss this band,
- you can’t/don’t want to do both.
Don't enter into any debate on these things, just be assured and present it as fact rather than possibility.
Make a special point of addressing B. he’s only just joined and has possibly invested a lot of confidence in the band and might feel that he’s joined up under false pretences. If there is some element of your own circumstances that have changed since he joined, that don’t relate to him, emphasise those. That way you underline to the rest of the band that you aren’t leaving because of B and B can understand that sometimes timing is what it is and no-one is to blame.
Then, I suggest that you leave them to get on with the practice and discussing what it means next for the band, don’t hang around, it isn’t your band any more.
If you want to stay in touch with your old bandmates, be prepared for the fact that they might need a little bit of time to be okay with things, don’t pressure them, but make sure they know where to find you if they feel friendly.