I don't play competitive, but I could still give you my opinion.
Lonely? Well, I don't really recommend that. Sylveon has more special attack than attack, and yours doesn't have any physical moves. Calm would be better, or maybe Sassy if you don't mind your Sylveon being pretty slow. Though you'd have to get another Sylveon for that.
I'd say get EVs for special defense and special attack. Those are Sylveon's highest base stats. I'm not sure what you'd want for the small extra EVs, probably either Defense or HP since those are left. Again, I'm not an expert on this kind of stuff, but I'm probably not too far off.
Bold nature is the best for any competitive sylveon imo; its special defenses are already fine, so bulking up on the physical defense is a necessity to help it wall a lot of sweepers, especially if it's a healer. the only other nature i would go with is Timid so it can help balance out the terrible base 60 speed stat for faster healing or w/e
in terms of EV spread, 252 HP / 252 Sp Def for sure
maxing out your physical defense could be really useful tho... hooray for super training
look for perf IV's in HP, def, spec attack, and spec defense. you can skip speed if you're not worried about it, but if you are... it's up to you. don't worry about physical attack; sylveon will never be a successful physical attacker. especially as a cleric lol
edit: also, this may seem like a silly idea, but if you're planning on using sylveon in doubles at all, it might be a good idea to replace hyper voice w draining kiss (or moonblast for extra power, but less healing), and get a sylveon with cute charm instead; even though draining kiss is a significantly weaker move, it still gets STAB and will also assist in healing sylveon so that sylveon can keep healing all of your other team members. and cute charm would be better defensively under this particular circumstance bc as long as the pokemon opposing sylveon is a diff gender and using physical moves (which would be the smartest strategy to knock sylveon out fast), 1/3 hits it'll be attracted to it, and thus 50% chance of not being able to move during their turn. if you IV in def and max def EV's, sylveon should be able to wall well enough to survive long enough to see cute charm come to fruition, and then draining kiss can sustain it in the mean time or wish will kick in or... you know... w/e you choose to do. lol. but this set up would be a lot more circumstantial than a pixelate cleric sylv.
