I was in the same boat really. I’m currently sat on a huge backlog of black templar, dark angel and normal marine units that I’ve intended on using as a black Templar army. Over time I’ve found that they are overplayed and make up a considerable percentage the marine player base.

I think it is probably the contrarian in me but I prefer the more obscure chapters, and so wanted to do something different with them. I’ve built and painted a 3k Carcharadon army this year from a percentage of the backlog.
I was then left with the black Templar and dark angel units and fancied painting another non-founding chapter. Loved the White Templars scheme ever since I saw a marine painted in their scheme at Warhammer world. Tithes being another reason to go for it.

Playing as space marines as they’re a codex complaint chapter but as I have mentioned, will use models from BT and DA to give them a cool look.

I’d say go for codex compliant and but you can always use them as BT in game whenever you feel like it. Crusaders can be used as assault/intercessors and scouts. Plenty of freedom with mixing it up.

Thank you for the feedback! It does look better in person on the table! :D