There are a lot of guides for you. Here is one example for you to do those: Guide: split-Protonmail (offline send/receive qubes + pm bridge vm)
It is required to in TemplateVMs for the icons to show persistently without any annoyance to reinstall over and over again. As such, PMB is certainly needed for your TB to receive them securely via the PMB. Here’s the article of why you need it: Why you need Proton Mail Bridge | Proton
In those articles, you really needed the PMB for your TB to receive your Proton Mails because Qubes is all about security with privacy in mind. Therefore, you shall need the PMB to receive your emails solely because of privacy and security; you don’t want anyone to spy on you. @ac.qubes-os