That's a really smart question to ask. A lot of people fall into this trap. My advice: keep your theme as simple and lightweight as possible. Use a clean, fast theme (like Blocksy, Kadence, or GeneratePress). Then, add functionality with high-quality, dedicated plugins. The problem with 'multi-purpose' themes is they try to be everything. They bundle dozens of features you'll never use, which slows your site down. It's much better to have a 'stack' you control. Get a good membership plugin (like MemberPress) and a good forum plugin (like bbPress). This way, if you ever want to change your site's look, you can just swap the theme, and all your functionality (from the plugins) stays. It's a much more flexible and professional way to build. Finding good wordpress plugins themes that work well together is the key. Are you more focused on the community (forum) or the paid content (membership) side?