I would definitely treat myself to a ring, because a wedding ring is so special, and because you see it every day. Especially because it is what you wear the most.
You might consider if there are places/ways to wear the pieces you don't wear much now. It might involve stepping outside your comfort zone, but it could be fun to try.
About sentimental pieces, I'm a mixed bag. Some things are sentimental, others I would sell. And if something is remade I could still remember where the pieces came from.
I'm glad you found a placeholder. I would definitely keep the older one. I would ask more than one jeweler for ideas on reworking it if that interests you. I have seen some wonderful "redos".
f you want to buy a new ring you could consider consignment stores or jewelers who specialize in older jewelry. The cost can be much less than what you find elsewhere. Which is why a jeweler may buy from someone for 25% of what they paid retail, and sell it for 50% of the retail price. I have sold some non-sentimental pieces, for a sentimental piece I was advised to see if one day I will have daughters-in-law who will like them, as I wouldn't get much for them, and the stones weren't worth reworking.
If, like me, you sometimes don't spend money on yourself, I would encourage you to not wait for your birthday, to start looking, and if you find something you love, buy it, assuming it is a price that won't cause financial problems for you.