If you’re determined to solo everything, then you need gadgets for survivability, and crowd control. Flame turret is only useful for choke points. I’d recommend a regular assault turret for most solo play, as it can suppress enemies, and essentially gives them a second attack vector to account for, so you can flank enemies. You’re not using the turret for it’s actual damage output - you’re using it to draw attention.
The heal chem launcher is a must for solo play - double tap the shoulder button it’s equipped to, to drop yourself a heal over time. The revive hive is also good for solo play, as it automatically deploys on you if you get downed. The Firefly, Shield, Drone, Pulse and Seeker Mine, are all pretty bad right now, in terms of relative usefulness compared to the chem launcher and turret. Virtually every end game player runs some combination of Heal Chem/Revive Hive/Flame Turret right now for a reason.
Lastly, a marksman rifle should NOT be your primary weapon if you’re solo. Especially one with an actual scope on it. You’re just asking for enemies to run up on you, which is probably why you’re finding it difficult. An SMG, AR, or LMG should be your primary gun - SMGs are good for short range, LMGs are good for spraying lots of bullets with less accuracy, and ARs are good for just about everything, which is why most people use them. Marksman rifles should just be for picking off the guys your primary weapon can’t efficiently dispatch.
If you like semi-auto weapons with range, consider trying a regular rifle too, although I hate them.
Find defensible positions and beeline for them, then prioritize flankers so you don’t get surrounded. Always manually destroy your turret after an encounter is over if it still has time left (hold the shoulder button down for a few seconds), as it will greatly reduce to cooldown to redeploy it. Prioritize attack stats on gear over virtually anything else.
And I know someone already asked you about this, but - matchmake. Having other people present to draw fire will allow you to snipe to your hearts content, and 99% of the groups I’ve been in never say a word over comms. The four of you just silently shoot stuff, then go your separate ways.