The least pain approach would be a second Mac running High Sierra, or Mojave, and Pages v8.1 with MathType 7. Vendors have had an extended timeframe to prepare 64-bit applications, so waiting for MathType 8 could be tedium.
Then things start to go downhill if you only have one Mac. You can go back to Mojave, or High Sierra — provided that you performed a last Time Machine backup prior to the upgrade, and that you did not reuse that same Time Machine drive to backup Catalina. In theory, you would boot into Recovery and select Restore from Time Machine Backup to wipe the Catalina drive and restore everything as it was before the upgrade. I say in theory as obviously, I have not done this with a retreat from Catalina.
If the second paragraph is a bust, you are looking at a clean install of a supported, prior operating system to replace Catalina, and without a backup, you will lose all data on the existing Catalina installation.
Apple's built-in, aenemic, equation solution is a C++ compiled translator that either passes through MathML, or translates a subset of [LaTeX] syntax into MathML objects in Pages v7.1 and later. It is a nail biting alternative to MathType, and does not address existing MathType equations, but does give one in-line equations. It does not support any [La]TeX packages, so without amsmath, it may frustrate more than reward.
