Careful when you ask for advice. By all means please do, this is why the forum exists, but also bear in mind that choices depend on personal and subjective preferences, experiences, rosters, philosophy, etc.
With that said, to add more to your confusion, here’s my $0.02. After a full year of game play, I think the best and quickest path for new players is to build the strongest and most complete mono team possible for each color and each level. I believe that if you achieve that, you basically have 99% of your game needs covered.
By a complete team I mean a combination of healers, good offensive heroes, good titan heroes, at least one very good defensive hero, buffers, de-buffers, dispellers, and other good special skills and talents. For example, my 4* blue team is Kiril, Sonya, Sonya costume, Grimm, and Triton.
An added benefit of this is that now, before maxing a hero, I look at the team I have, or envision to have, for the same color and level as that hero, and then I decide if the new hero is a good fit.
So for example, looking at your blue options, between Richard (good defensive hero, very good tank, decent offensive hero), Vela (good hero all around), Isarnia (very good titan hero, decent offensive hero, mediocre on defense), and Thorne (decent to mediocre), I would ask which one completes or improves my 5* blue team and in my case, based on my roster, the answer would be Vela as I need more power on offense and titans. But you may easily chose something else and I guarantee you there are no bad choices. In the end, YOU are building YOUR team, the way YOU like it, and with what YOU have.
Hope I helped more than I confused you.