Been looking to craft Reno and a few other cards for Highlander since before the rotation but never got the chance, and now I am only missing Reno the Relicologist and Astromancer Solarian. Should I go for the Highlander version and craft these 2 or just stick to Spell Mage for now?
Spell Mage is very good RIGHT NOW. You can't know for sure if it will be good next week. It all depends on the meta shift after DH nerfs (and after people get fed up with playing it).
Another thing is Highlander is still good vs DH. I won't give you any percentages, general consensus is that Spell Mage is better vs DH than Highlander, but they are both pretty much favorable match-ups. At least from my experience.
Also keep in mind Reno is an universal cards. Many classes have viable highlander variants, including newly introduced DH. You can't really go wrong with crafting it.
I wouldn't recommend crafting Reno mage right now unless you're big into wild, where Reno the relicologist is an excellent card. In standard if you want to play mage just stick with spell mage, highlander mage is a frustrating deck to play in standard since it's pretty weak versus aggro and a lot of games are decided by things out of your control.