You have every right to start monetizing the site. But the ads in the middle of a thread are sincerely obnoxious. I'd recommend putting them as the last post rather than smack dab in the middle of the thread.
With respect, at this point, just call it a membership and not a donation. If that's the only way to turn off ads, and the ads are meant to drive profits, just charge for a membership already. You've been walking a line resisting charging for membership because it's a hobby site - then institute these ads which are obnoxious and only way to remove is donation plus. Again, respectfully, you crossed the line you've been walking and it's ok. Time to call it what it is - a membership.
Another recommendation: Build a mobile app for viewing the site and charge something for it. The site is painful to view form a mobile phone at times.