What do you need, e.g. what did the last registrar get wrong? We built our own system from the ground up, and were fairly early in the game so we didn't copy anyone else. Much of our stuff is bulk friendly. e.g. once a month renewal reminder that covers all your .ca domains in the next few months. A "paste a list of domains here" and a bunch of forms below for bulk updates. And conversely, there are things that are a little rough or just "different".
What we don't do well is non .CA domains. We're just a tucows reseller for those, I've been less and less happy with them over the years. (Awesome at the beginning, but have gone downhill like Google in the name of profit.)
I obviously have minimal experience with other registrars, and I haven't asked what it costs to be a sponsor (if you have to ask, you can't afford it), but with the rare exception of clearly marked specials, renewals have always been the same as registrations at baremetal. I can't morally wrap my head around sucking people in with loss-leaders / lowballs and then doing the add-on thing.
(bulk discounts are calculated separately though, so folks can go through a period where their discounted new registrations may be cheaper than their first renewals.)
(My apologies for the non-TBR / spammy post.) -Tom