Usually the higher up you can go, the better. Have you tried your current antenna in the attic? Just raising it may improve your reception.
If moving your current antenna to the attic helps, but doesn’t eliminate your problems, then yes a better antenna might help. I currently have an Antop 400-BV on order to replace a 30 year old yagi that is mounted on a tower next to my house. Since I haven’t tested it yet, I can’t say if it is as good as or better than the old yagi. Hopefully I’ll know in a few days, but it does have excellent reviews especially from fellow Tablo users.
The longer the cable the more there will be loss in signal. It also matters if there are splitters in the existing line and what type of cable is used. RG-59 cable has more loss per foot than a quad shielded RG-6 cable. I’m currently using 35 year old RG-59, but plan to replace it with RG-6.
Sorry I don’t have any experience with antenna installation companies, I do all of my own work.
Good luck. Having a strong and stable antenna signal is vital to enjoying the OTA experience.