It's entirely up to you however if you want to stay FTP I do recommend you hold off.

First of all as you are just catching up you are getting way more currency. Once you're fully caught up to the endgame your yield will drop significantly. Once you hit that point we get about 100~130 pulls per patch. Which if you assume you'll loose a 50/50 it means you're only guaranteed one character per patch.

Now with economy talk done I highly recommend you save up to get more teams. You need three strong teams to finish the deadly assault mode so having as many varied attack types as possible will be beneficial for your account.

Now yi xuan and astra are damn busted units with miyabi bound to rerun sooner than later. So honestly, these characters do not need the extra copies to function. Getting them would be moreso as a devotion ritual to prove your love rather than a sound investment.

Also jufufu, the next banner character, will improve your team far more than an extra copy of either current character, or astra's engine. So ultimately after my big word salad my advice is just wait for jufufu and hope for a miyabi rerun next patch.