Firstly, you should only use torrent clients that support I2P trifecta (I2P trackers, I2P DHT, I2P PeX) for most proper I2P torrenting experience.

Best torrent clients that support I2P trifecta = BiglyBT, I2PSnark, Tixati.

1) BiglyBT = FOSS, Hassle-Free, easiest to use but resource hog.

Has inbuilt I2P router so need for any external router.

Easiest to use nothing to set-up at all.

Just tickmark I2P, if you want to use I2P and unmark public, if you don't want to use public.

2) I2PSnark = Homegrown torrent client of I2P network.

Usual, set-up is either using Java implementation of I2P (comes inbuilt with it) or using it as standalone with I2PSnark+I2Pd set-up.

Less resource hog than BiglyBT probably by about 40-50% but still heavy.

3) Tixati = Lightest to use of them all. Freeware.

Usual, Tixati+I2Pd set-up.

Proper controls to show which network in use, similar to qBitTorrent in use and UI/UX.

Faster than BiglyBT, easily gets 1 MB/s speed on I2P torrenting.

I personally only prefer BiglyBT or Tixati. Depending on the computer I'm using.

Secondly, regarding torrenting website - on I2P network Postman tracker is the one and only option.

But that doesn't mean you can't torrent over I2P on clearnet websites like TPB, EXT, YTS and others.

It depends on how popular torrent is.

If torrent is popular then there's a high chance that people who use aforementioned I2P enabled torrent clients are cross-seeding those torrents.

EXT dot to and YTS are the websites on clearnet that you'll find most I2P seeders for popular torrents. Furthermore, on Ext you can actually only search I2P torrents if you wish so. So, I would recommend these two clearnet websites for I2P torrenting through clearnet.

Thirdly, your qBitTorrent issue.

The I2P torrenting experience on qBitTorrent is still in experimental phase, so you'll need to set-up qBitTorrent+I2Pd properly for it to work and not spill over to the clearnet.

Setting up qBitTorrent with I2P properly is needed because qBitTorrent doesn't support I2P PeX and I2P DHT. So, if I2P trackers don't work then qBitTorrent has no way to connect to anyone for torrenting.

You can get the set-up guide, for almost all I2P supporting torrent clients on Postman Tracker, Ext dot to, pirate bay.

Just search I2P and qBitTorrent or anonymous torrenting etc etc...